Revert "install: unlink old kegs"

This reverts commit 2eabe2cbc84649696aeb6fa842a70f3794955597.

When Keg#unlink looks for symlinks relative to the keg, it can hit false
positives that actually belong to a different keg and unlink them
anyway. This breaks our "force identical directory symlinks to be shared
real directory" case.

This may be a problem in general with the unlinking code and should be
investigated.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/install.rb

@@ -103,10 +103,6 @@ module Homebrew extend self
         begin
           fi = FormulaInstaller.new(f)
           fi.install
-          # Due to the nature of Keg#unlink, this will remove symlinks from an
-          # older keg, which may still be present if an uninstallation was done
-          # via `rm -rf <keg>`; this is desired.
-          Keg.new("#{f.rack}/#{f.version}").unlink
           fi.caveats
           fi.finish
         rescue FormulaAlreadyInstalledError => e