Prune excessive builtin columns

[rrnewton]

Since the schema has become extensible, the core set can shrink down in the 2.0 release, where all the breaking changes are being put.

[rrnewton]

Here's an initial proposal for what to nix and what to keep

Keep:

   _PROGNAME 
   _VARIANT  
   _ARGS     
   _HOSTNAME 
   _RUNID    
   _CI_BUILD_ID 
   _THREADS  
   _DATETIME 
   _MINTIME    
   _MEDIANTIME 
   _MAXTIME
   _ALLTIMES
   _TRIALS
   _COMPILER      
   _COMPILE_FLAGS 
   _RUNTIME_FLAGS
   _UNAME      
   _PROCESSOR  
   _GIT_BRANCH 
   _GIT_HASH   
   _GIT_DEPTH
   _WHO          
   _RETRIES 
   _CUSTOM 

Cut:

   _MINTIME_PRODUCTIVITY    
   _MEDIANTIME_PRODUCTIVITY 
   _MAXTIME_PRODUCTIVITY    
   _ENV_VARS      
   _BENCH_VERSION 
   _BENCH_FILE 
   _TOPOLOGY   
   _ETC_ISSUE  
   _LSPCI      
   _FULL_LOG   
   _MEDIANTIME_ALLOCRATE    
   _MEDIANTIME_MEMFOOTPRINT 
   _ALLJITTIMES   

Opnions? @svenssonjoel, @osa1, @adk9, @vollmerm, @eholk, @chamibuddhika -- you've all used hsbencher, what do you think?