Give up on testing guc.c's behavior for "infinity" inputs.

Give up on testing guc.c's behavior for "infinity" inputs.

Further buildfarm testing shows that on the machines that are failing
ac75959cd's test case, what we're actually getting from strtod("-infinity")
is a syntax error (endptr == value) not ERANGE at all.  This test case
is not worth carrying two sets of expected output for, so just remove it,
and revert commit b212245f9's misguided attempt to work around the platform
dependency.

Discussion: https://postgr.es/m/E1h33xk-0001Og-Gs@gemulon.postgresql.org