Remove xfail in gdb.mi/mi-watch.exp

Remove xfail in gdb.mi/mi-watch.exp

I noticed there was an unexpected pass in mi-watch.exp when running on
x86_64.  Doing a bit of archeology shows that the xfail was added by
4a543da.  This particular test failed on the MIPS architecture, which
the original contributor was working with.  Here is the thread:

https://www.sourceware.org/ml/gdb-patches/2007-09/msg00151.html

Looking at the latest buildbot results for MIPS, it seems that it's also
an unexpected pass on that architecture.  Therefore, I see no reason to
leave the xfail in place.

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-watch.exp (test_watchpoint_triggering): Remove xfail.