Unnecessary restore of timeout global at end of gdb.base/completion.exp

This patch removes some code in gdb.base/compilation.exp which
is aimed at restoring the original timeout global value after having
changed it for this testcase. Restoring the timeout global is not
necessary as this is taken care of by gdb_init, which is called
at the start of each testing.

gdb/testsuite/ChangeLog:

        * gdb.base/completion.exp: Remove code aimed at restoring TIMEOUT.