Timer: Remove group-less NamedRegionTimer constructor.

The NamedRegionTimer initializer without a group name puts the Timer
into the "Misc" group and is (nearly) unused. Remove it.

The only user of this constructor appears to be the HexagonGenInsert pass,
which creates a counter without group to count the complete execution
time of that pass, however since every pass gets a counter by the
PassManager anyway this should be unnecessary. Also removed the
pointless TimerGroup there.

Differential Revision: https://reviews.llvm.org/D25582

llvm-svn: 286524