[BOLT] Fix icp-top-callsites option, remove icp-always-on.

Summary: The icp-top-callsites option was using basic block counts to pick the top callsites while the ICP main loop was using branch info from the targets of each call.  These numbers do not exactly match up so there was a dispcrepancy in computing the top calls.  I've switch top callsites over to use the same stats as the main loop.  The icp-always-on option was redundant with -icp-top-callsites=100, so I removed it.

(cherry picked from FBD6370977)