gdb/testsuite: remove MPFR detection in gdb.base/float128.exp

I see this fail since commit 99118062 ("Use toplevel configure for
GMP and MPFR for gdb"):

    FAIL: gdb.base/float128.exp: show configuration

The test fails to find --with-mpfr or --without-mpfr in the "show
configuration" output.  Since MPFR has become mandatory, we can just
remove that check and simplify the test to assume MPFR support is there.

Change-Id: I4f3458470db0029705b390dfefed3a66dfc0633a
Approved-By: Tom de Vries <tdevries@suse.de>