cpu: Get rid of the unused IsMicroBranch StaticInst flag.

This flag was never set, nor read.

Change-Id: I74506c220d96b53dcd44740639286b1dbbe84d2e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33742
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/cpu/StaticInstFlags.py

@@ -99,9 +99,6 @@ class StaticInstFlags(Enum):
         'IsDelayedCommit',  # This microop doesn't commit right away
         'IsLastMicroop',    # This microop ends a microop sequence
         'IsFirstMicroop',   # This microop begins a microop sequence
-        # This flag doesn't do anything yet
-        'IsMicroBranch',    # This microop branches within the microcode for
-                            # a macroop
 
         'IsSquashAfter',     # Squash all uncommitted state after executed