Remove bug48187.phpt

Saw a spurious failure from this one on azure macos, presumably
the process got interrupted in the middle and waited for more than
one second to resume.

After looking a bit more closely, this test doesn't actually test
what bug #48187 reported, because there is no DateTime::diff()
anywhere to be found. This test was also added speculatively,
because the root cause was never diagnosed, and the problems seems
to have resolved itself at some point.

As such, I'm simply dropping this test, rather than figuring out
how to make it more robust.

Author: nikic