Revert "[global] Use realpath in case HOMEBREW_PREFIX is a symlink"

This reverts commit 6d3afa994c8b3fc528d8672fa307a02404b55bf2.

Leave HOMEBREW_PREFIX unmolested because it must indicate the logical
location of installation to accommodate packages which are tied to
particular locations (eg /usr/local).
[keg] Use HOMEBREW_PREFIX.realpath when constructing symlinks

https://github.com/Homebrew/homebrew/issues/26524

HOMEBREW_PREFIX is set to the parent of the dirname of HOMEBREW_BREW_FILE.
In those cases where HOMEBREW_PREFIX is a symlink, where keg.rb
is constructing relative symlinks into HOMEBREW_PREFIX,
use HOMEBREW_PREFIX.realpath to follow the underlying algorithm used by
the operating system.