std.datetime: eliminate _assertPred

The 32 bit build started to crash on my Mac and the 64 bit build took a long time, so I eliminated _assertPred. Numbers:

BEFORE

```
/usr/bin/time -l -- ../dmd/src/dmd -I../druntime/import  -w -m32  -O -release -unittest -c -ofgenerated/osx/release/32/unittest/std/datetime.o -deps=generated/osx/release/32/unittest/std/datetime.deps.tmp std/datetime.d
Command terminated abnormally.
        9.74 real         8.97 user         0.56 sys
1236336640  maximum resident set size
         0  average shared memory size
         0  average unshared data size
         0  average unshared stack size
    303144  page reclaims
         0  page faults
         0  swaps
         0  block input operations
         6  block output operations
         0  messages sent
         0  messages received
         0  signals received
         0  voluntary context switches
       159  involuntary context switches
```

```
/usr/bin/time -l -- ../dmd/src/dmd -I../druntime/import  -w -m64  -O -release -unittest -c -ofgenerated/osx/release/32/unittest/std/datetime.o -deps=generated/osx/release/32/unittest/std/datetime.deps.tmp std/datetime.d
        7.88 real         7.30 user         0.55 sys
1246662656  maximum resident set size
         0  average shared memory size
         0  average unshared data size
         0  average unshared stack size
    305962  page reclaims
         0  page faults
         0  swaps
         0  block input operations
         0  block output operations
         0  messages sent
         0  messages received
         0  signals received
         0  voluntary context switches
       124  involuntary context switches
```

AFTER

```
/usr/bin/time -l -- ../dmd/src/dmd -I../druntime/import  -w -m32  -O -release -unittest -c -ofgenerated/osx/release/32/unittest/std/datetime.o -deps=generated/osx/release/32/unittest/std/datetime.deps.tmp std/datetime.d
        5.86 real         5.55 user         0.29 sys
 614715392  maximum resident set size
         0  average shared memory size
         0  average unshared data size
         0  average unshared stack size
    151683  page reclaims
         0  page faults
         0  swaps
         0  block input operations
         3  block output operations
         0  messages sent
         0  messages received
         0  signals received
         0  voluntary context switches
       139  involuntary context switches
```

```
/usr/bin/time -l -- ../dmd/src/dmd -I../druntime/import  -w -m64  -O -release -unittest -c -ofgenerated/osx/release/32/unittest/std/datetime.o -deps=generated/osx/release/32/unittest/std/datetime.deps.tmp std/datetime.d
        5.47 real         5.18 user         0.28 sys
 612421632  maximum resident set size
         0  average shared memory size
         0  average unshared data size
         0  average unshared stack size
    151290  page reclaims
         0  page faults
         0  swaps
         0  block input operations
         1  block output operations
         0  messages sent
         0  messages received
         0  signals received
         0  voluntary context switches
        96  involuntary context switches
```