Bug 97954: Delete SpiderMonkey's custom build system for separate builds. r=bsmedberg

SpiderMonkey has, at present, two separate build systems: one used for
in-tree builds, and one used for SpiderMonkey-only builds.

The former is constituted entirely by js/src/Makefile.in.  This is a
typical Mozilla makefile, leaning on the top-level config/*.mk files
for most of the details.

The latter is comprised by a shadow build system, all in js/src:
Makefile.ref, config.mk, rules.mk, jsconfig.mk, and config/*.mk.
These are superseded by the separate build system implemented in a
later patch; we delete them all here.

This patch consists entirely of deletions; there are no textual changes.