Remove non-standard opcodes

[DavidKinder]

Git has two non-standard Glulx opcodes, git_setcacheram (0x7940) and git_prunecache (0x7941). These are never, to my knowledge, used. It seems wrong to have unused non-standard opcodes, so these should be removed.

The Glulx specification says

Opcodes 0x7900 to 0x79FF are (apparently) reserved for experimental features in the Git interpreter.

Once these opcodes are gone, a change should be pushed to the specifications repository to change this to something historical, e.g.

Opcodes 0x7940 and 0x7941 were available for cache control in the Git interpreter, however this is no longer supported and these opcodes should not be considered to be reserved.