Fix unused vars etc. in internal Lua files.

Thanks to François Perrad.

src/jit/dis_mips.lua

@@ -11,8 +11,8 @@
 ------------------------------------------------------------------------------
 
 local type = type
-local sub, byte, format = string.sub, string.byte, string.format
-local match, gmatch, gsub = string.match, string.gmatch, string.gsub
+local byte, format = string.byte, string.format
+local match, gmatch = string.match, string.gmatch
 local concat = table.concat
 local bit = require("bit")
 local band, bor, tohex = bit.band, bit.bor, bit.tohex