AthenaAuditors + PerfMonGPerfTools: No longer turn off the build of AthenaAuditors and PerfMonGPerfTools in the absence of GPerfTools
As discussed in ATEAM-654, with this MR we no longer turn off the AthenaAuditors and PerfMonGPerfTools build in the absence of gperftools. The original workaround was needed because the clang8 build of LCG_96 didn't provide gperftools, which is not relevant any longer. Also, it overshadows problems w/ gperftools in general.