Prune INSTALL file

along the same line of thought as #24363 (moved), literally nobody needs to run setenv LD_LIBRARY_PATH /usr/athena/lib anymore. hell, basically nobody runs setenv at all anymore.

Also, does anybody actually statically compile tor using the method specified? I would assume that most people use the distro packages. even TB doesn't compile static, at least on Linux.

I figure we can dump everything but the first two paragraphs, and maybe add a reference to doc/building-tor-* (#26376 (moved)).

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