Remove main_js_comp.ml and main_hhbc_comp.ml.

Summary: Verified with@pikatchu@joelbeales and@jdelong that those files
were no longer necessary. The question arose because those files invoke
Pos.make, whose call signature I recently changed.

Reviewed By: @jdelong

Differential Revision: D1678179

Signature: t1:1678179:1415894809:ae35c4c1f6c2bc196096379e905d2faead5bb503