Remove non-Homebrew-specific shell caveats (#43107)

* bash: remove non-Homebrew-specific caveats

* fish: remove non-Homebrew-specific caveats

* mksh: remove non-Homebrew-specific caveats.

Formula/bash.rb

@@ -52,12 +52,6 @@ class Bash < Formula
     system "make", "install"
   end
 
-  def caveats; <<~EOS
-    In order to use this build of bash as your login shell,
-    it must be added to /etc/shells.
-  EOS
-  end
-
   test do
     assert_equal "hello", shell_output("#{bin}/bash -c \"echo -n hello\"")
   end

Formula/fish.rb

@@ -40,17 +40,6 @@ class Fish < Formula
     (pkgshare/"vendor_conf.d").mkpath
   end
 
-  def caveats; <<~EOS
-    You will need to add:
-      #{HOMEBREW_PREFIX}/bin/fish
-    to /etc/shells.
-
-    Then run:
-      chsh -s #{HOMEBREW_PREFIX}/bin/fish
-    to make fish your default shell.
-  EOS
-  end
-
   test do
     system "#{bin}/fish", "-c", "echo"
   end

Formula/mksh.rb

@@ -17,13 +17,6 @@ class Mksh < Formula
     man1.install "mksh.1"
   end
 
-  def caveats; <<~EOS
-    To allow using mksh as a login shell, run this as root:
-        echo #{HOMEBREW_PREFIX}/bin/mksh >> /etc/shells
-    Then, any user may run `chsh` to change their shell.
-  EOS
-  end
-
   test do
     assert_equal "honk",
       shell_output("#{bin}/mksh -c 'echo honk'").chomp