Remove "__brew_ps1" function from completion

This was probably silly and I'm probably the only person that ever used
it. I still do, actually, but it's not really a completion function,
it's simple enough to just stick in a shell startup script, and removing
it makes the completion script usable under the new dynamic loading
scheme provided by bash-completion 1.99+.

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

Library/Contributions/brew_bash_completion.sh

@@ -10,30 +10,10 @@
 #    $(brew --prefix)/etc/bash_completion.d
 #    $(brew --prefix)/share/bash-completion/completions
 #
-# and bash-completion will source it automatically.
-#
-# The __brew_ps1() function can be used to annotate your PS1 with
-# Homebrew debugging information; it behaves similarly to the __git_ps1()
-# function provided by the git's bash completion script.
-#
-# For example, the prompt string
-#
-#     PS1='\u@\h \W $(__brew_ps1 "(%s)") $'
-#
-# would result in a prompt like
-#
-#    user@hostname cwd $
-#
-# but if you are currently engaged in an interactive or debug install,
-# (i.e., you invoked `brew install` with either '-i' or '-d'), then the
-# prompt would look like
-#
-#     user@hostname cwd (<formula_name>|DEBUG) $
-#
-# You can customize the output string, e.g. $(__brew_ps1 "[%s]") would
-# output "[<formula_name>|DEBUG]". The default (if you do not provide a
-# format argument) is to print "(<formula_name>|DEBUG)" prefixed with a
-# single space.
+# Installing to etc/bash_completion.d will cause bash-completion to load
+# it automatically at shell startup time. If you choose to install it to
+# share/bash-completion/completions, it will be loaded on-demand (i.e. the
+# first time you invoke the `brew` command in a shell session).
 
 __brewcomp_words_include ()
 {
@@ -375,12 +355,6 @@ _brew_versions ()
     __brew_complete_formulae
 }
 
-__brew_ps1 ()
-{
-    [[ -n $HOMEBREW_DEBUG_INSTALL ]] &&
-    printf "${1:- (%s)}" "$HOMEBREW_DEBUG_INSTALL|DEBUG"
-}
-
 _brew ()
 {
     local i=1 cmd