Remove some PHP4 microtime() compat wrappers

Trivially replaced with microtime( true ).

The following classes still use microtime() without the
argument, though I have chosen not to change them in this
commit because of a possible loss of precision:

* MWCryptHKDF
* UIDGenerator
* UpdateSpecialPages
* UploadStash

Change-Id: I4f594f6ada9e8fce3319b61844b9857d87e9aa99
Merge "Remove some PHP4 microtime() compat wrappers"