lua: stop trying to tweak lua's GC

cherry-picked from f0ca89ac, original author Vlada Cunat

TL;DR: I believe all lua_gc() calls stemmed from misunderstanding lua
documentation, and the current settings seem potentially dangerous.

First, let me rely on lua 5.1 docs, as luajit 2 is documented to have
done only minor changes in the GC.
http://www.lua.org/manual/5.1/manual.html#lua_gc
http://wiki.luajit.org/New-Garbage-Collector#rationale

Commit 5a709411 claims to have increased the speed of GC to 400 % of
speed of allocation, but LUA_GCSETSTEPMUL is the parameter that
controls that, and that one was lowered to 99 % and later in
0ee2d1d7 even to 50 %.  Documentation explicitly says that setting
the value under 100 % may cause problems.

The default values seem perfectly sane to me and currently I can't see
any particular reason to change them.  It's 200 % relative GC speed,
and waiting for allocated size to double before starting another cycle.

I assume the resulting possibility of GC being too slow caused the need
to explicitly force a non-incremental GC cycle once in a while, but
that seems not useful anymore and not good for latency.