Nit - replace computing constant with that constant

`fontforge/parsettf.c :: readdate()` was computing the time difference between the 1904 epoch time ala Mac and the 1970 epoch time ala Unix. However this value is a constant and need not be recomputed each time. One could inspect the computed value once and make static, or derive the value from various references on the web.

This change replaces code and loop with initialization of a variable with the constant value.

Tested against three different ttf/ttc files and verified from the  "Element / Font Info... / Dates" displays before and after change.
The difference in seconds btw. 1904-01-01 and 1970-01-01, but split up into the 16-bit segments as needed by the loop following the deleted code.   (31781 * 65536) + 45184  =  2082844800    which  / 86400 secs  =  24107 days   which includes the leap days for 17 leap years btw. those years.  I grabbed the 16-bit values as computed by the replaced loop, in addition to verifying that 2082844800 was the correct magic number against multiple discussions on the web.