Enumerated constants CODE_AGE_MARKER_NOP and CODE_AGE_MARKER_NOP were
defined for using in code aging.
Support for code aging is removed with following:
  8bf15bf1
    Reland "[Compiler] Remove code aging support.".
After this, mentioned constants are not used anymore in MIPS
implementation and therefore they can be removed.

TEST=
BUG=

Change-Id: Ica2b3ceeb94a8fc1110760beff159e82024d3ed3
Reviewed-on: https://chromium-review.googlesource.com/966183
Commit-Queue: Ilija Pavlović <ilija.pavlovic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#51983}