Utilities/Release: Drop unnecessary system API definitions for Linux

We previously defined _POSIX_C_SOURCE and friends while building binary packages in order to minimize the version of glibc needed at runtime. The definitions were added by !3727 (merged), but came from older packaging scripts that were removed by the same MR. Those older scripts were meant for use in a hand-maintained environment. Now that we use base images of old CentOS versions to establish the build environment, our builds are already limited to older glibc versions (glibc 2.12 from centos6 on x86_64, and 2.17 from centos7 on aarch64).

Our old system API definitions no longer affect the glibc version required by the binaries. Drop them to avoid potential conflicts with system API definitions added by changes like !4477 (merged) and !7320 (merged).