Remove conditional on LDBL_MANT_DIG from e_lgammal_r.c

The IEEE 754 implementation of lgammal in sysdeps/ieee754/ldbl-128/ used
to be shared by IBM's implementation in sysdeps/ieee754/ldbl-128ibm/ (by
an inclusion of the source file).  In order for the algorithm to work
for IBM's implementation, a check for LDBL_MANT_DIG was required. Since
the source file is no longer shared, the requirement for the check is
gone.  This patch removes the conditionals.

Tested for powerpc64le and s390x.

	* sysdeps/ieee754/ldbl-128/e_lgammal_r.c (__ieee754_lgammal_r):
	Remove conditionals on LDBL_MANT_DIG.
	* sysdeps/ieee754/ldbl-128ibm/e_lgammal_r.c
	(__ieee754_lgammal_r): Likewise.

(cherry picked from commit 9ac3c682)