remove HAS_LDBL_SPRINTF_BUG code

remove HAS_LDBL_SPRINTF_BUG code

This code was added in 2002 to work round an Irix 6 rounding bug in
long double sprintfs.

I strongly suspect that any such OS bug has long been fixed and/or such
machines have been retired or are unlikely to have new perls installed on
them.

Part of the motivation for removing this code is that following the
previous commit, that block of code's use of the float_need variable
is likely to be wrong (since it now includes exponent etc), but I have no
way of testing it.

I've left the probe code in hints/irix_6.sh, so if anyone ever reports
sprintf.t failures on an old Irix platform, perl -V should show if their
system still has the bug. At that point someone brave could resurrect this
block of code.