Eliminate integer destination modifier.

Eliminate integer destination modifier.

The integer modifier was used to emulate integer arithmetic using
floating-point operations, as was allowed/typical for OpenGL ES 2.0
implementations. Now that we support native 32-bit integer types and
we have separate opcodes for integer operations, it is not longer
needed.

Change-Id: I89987534c150d2426ac9f9e1e49b66f9deaee560
Reviewed-on: https://swiftshader-review.googlesource.com/13889
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>