MIPS: Drop unnecessary code for R6 in interpreter (NAN2008)

The latest MIPS64R6 emulator supports NAN2008 standard. Because
of that many workarounds can be removed in conversion functions
in interpreter.

Test: ./testrunner.py --target --interpreter --64 in QEMU for MIPS64R6
Test: ./testrunner.py --target --interpreter --32 in QEMU for MIPS32R6
Test: ./testrunner.py --target --interpreter --32 in QEMU for MIPS32R2

Change-Id: Ibee66db7044c4110a4eace0187207c63ffe84629