`str_format/convert_test`: Delete workaround for [glibc bug](https://sourceware.org/bugzilla/show_bug.cgi?id=22142)

RHEL7 ships an affected version of glibc, but this bug is specific to powerpc, which abseil [does not support](https://github.com/google/oss-policies-info/blob/4ec9f2a979a6c6b7bb148040347c8d2b1def09fa/foundational-cxx-support-matrix.md).

PiperOrigin-RevId: 626018125
Change-Id: I1875aee6c37e69f712ef91e41da455af9d00fb5f