[mips][mips64]: Remove unused code aging constants.

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}

src/mips/assembler-mips.h

@@ -674,9 +674,6 @@ class Assembler : public AssemblerBase {
     // Helper values.
     LAST_CODE_MARKER,
     FIRST_IC_MARKER = PROPERTY_ACCESS_INLINED,
-    // Code aging
-    CODE_AGE_MARKER_NOP = 6,
-    CODE_AGE_SEQUENCE_NOP
   };
 
   // Type == 0 is the default non-marking nop. For mips this is a

src/mips64/assembler-mips64.h

@@ -681,9 +681,6 @@ class Assembler : public AssemblerBase {
     // Helper values.
     LAST_CODE_MARKER,
     FIRST_IC_MARKER = PROPERTY_ACCESS_INLINED,
-    // Code aging
-    CODE_AGE_MARKER_NOP = 6,
-    CODE_AGE_SEQUENCE_NOP
   };
 
   // Type == 0 is the default non-marking nop. For mips this is a