Remove MMTK_PLAN and THIRD_PARTY_HEAP_LIMIT

[chrisseaton]

We don't need to support these environment variables anymore now we have the flags, do we? Ruby lets you set options using the RUBYOPT environment variable, so they're not even needed to set globally.

[wks]

That's right. As we discussed in another issue (mmtk/mmtk-core#636), we will eventually remove the ability to set options via MMTK_xxxx environment variables.

Keeping RUBYOPT is important, as it is necessary for setting MMTk plans for make btest.