doctor: remove stale OS version check

Closes Homebrew/homebrew#19959.

Library/Homebrew/cmd/doctor.rb

@@ -1035,26 +1035,6 @@ def check_for_unlinked_but_not_keg_only
   end
 end
 
-def check_os_version
-  if MACOS_FULL_VERSION =~ /^10\.6(\.|$)/
-    unless (MACOS_FULL_VERSION == "10.6.8")
-      return <<-EOS.undent
-        Please update Snow Leopard.
-        10.6.8 is the supported version of Snow Leopard.
-        You are still running #{MACOS_FULL_VERSION}.
-      EOS
-    end
-  elsif MACOS_FULL_VERSION =~ /^10\.5(\.|$)/
-    unless (MACOS_FULL_VERSION == "10.5.8")
-      return <<-EOS.undent
-        Please update Leopard.
-        10.5.8 is the supported version of Leopard.
-        You are still running #{MACOS_FULL_VERSION}.
-      EOS
-    end
-  end
-end
-
   def check_xcode_license_approved
     return if MacOS::Xcode.bad_xcode_select_path?
     # If the user installs Xcode-only, they have to approve the