Remove `sly-recompile-bytecode'

Emacs 24.4 `load-prefer-newer' variable, the hotspot compilation and
the advent of Makefile take care of the relevant use cases for this
functionality.

* sly.el (sly-bytecode-stale-p, sly-recompile-bytecode)
(sly-urge-bytecode-recompile): Removed.
(sly-start): Don't recompile sly.el.