Temporarily remove InstOpCode::length

We temporarily remove this API because of a naming conflict in
`TR_X86OpCode` class. To avoid having to change the name of this
function for purposes of this work we simply temporarily delete the
API in `InstOpCode` base class until we have fully migrated
`TR_X86OpCode`, at which point we can reinstate the original API.

compiler/codegen/OMRInstOpCode.cpp

@@ -21,12 +21,6 @@
 
 #include "compiler/codegen/InstOpCode.hpp"
 
-uint8_t
-OMR::InstOpCode::length(Mnemonic m)
-   {
-   return 0;
-   }
-
 uint8_t *
 OMR::InstOpCode::copyBinaryToBuffer(
       Mnemonic m,

compiler/codegen/OMRInstOpCode.hpp

@@ -61,11 +61,6 @@ class InstOpCode
 
    static int32_t getNumOpCodes() { return NumOpCodes; }
 
-   /*
-    * Length of the encoded binary representation of an opcode.
-    */
-   static uint8_t length(Mnemonic m);
-
    /*
     * Copies the encoded binary representation of a given mnemonic to the
     * provided buffer.  This will copy at most length() bytes.