Remove lean4-lake.el in favor of Emacs built-in compile command

[mekeor]

@bustercopley says:

I [...] wonder if anyone would notice if we deleted "lean4-lake.el". We could just mention in the README that building happens automatically when lake is started as a server, and direct users to the compile command if they do need to run lake build.

Let's investigate this suggestion and then act.

[mekeor]

If we decide to not delete lean4-lake.el, then we should open an issue on the topic of using locate-dominating-file instead of defining the function lean4-lake-find-dir-in.