Drop questionable CLT advice

If an outdated Xcode is installed, it really needs to be updated if
possible. Otherwise things that depend on Xcode at build time will have
that dependency satisfied, but will fail to compile. The CLT is enough
if it is installed by itself, in which case Xcode dependencies will not
be satisfied.

Library/Homebrew/cmd/doctor.rb

@@ -255,9 +255,6 @@ if MacOS.version >= "10.9"
         s += <<-EOS.undent
           Xcode 6.1 is required on Yosemite to compile anything (6.0.1 does not include
           the 10.10 SDK). You can install it from the App Store.
-
-          For some software installing the CLT may be enough:
-            xcode-select --install
         EOS
       else
         s += "Xcode can be updated from the App Store."