DiffHistoryBlob: don't check for mhash extension

Checking for the mhash extension is pointless in PHP 5.3+ because
it only exists as B/C functions that are part of the hash extension.

From ext/hash/config.m4:

    if test "$PHP_MHASH" != "no"; then
      if test "$PHP_HASH" = "no"; then
        PHP_HASH="yes"
      fi

      AC_DEFINE(PHP_MHASH_BC, 1, [ ])
    fi

So if mhash() is compiled in, hash() must also be compiled in.

Change-Id: Ic376f4c438aaf701ebb4f1394ab6e4c1803fb8b8