configure: cleanup -fno-pie detection

Place it only inside the 'if test "$pie" = "no"' conditional.  Since
commit 43924d1e ("pc-bios/optionrom: detect -fno-pie", 2022-05-12),
the PIE options are detected independently by pc-bios/optionrom/Makefile,
and the CFLAGS_NOPIE/LDFLAGS_NOPIE variables are not used anymore.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>