Revert "cram.vim: use @shell from syntax/zsh.vim for SHELL=zsh"

This reverts commit f18fe33.

This was causing an issue where zsh.vim would fold all sections by
default. This change is also questionable considering one might be using
zsh locally but may be writing tests to run against /bin/sh.

contrib/cram.vim

@@ -15,11 +15,7 @@ if exists("b:current_syntax")
   finish
 endif
 
-if fnamemodify($SHELL, ':t') == 'zsh'
-  syn include @Shell syntax/zsh.vim
-else
-  syn include @Shell syntax/sh.vim
-endif
+syn include @Shell syntax/sh.vim
 
 syn match cramComment /^[^ ].*$/ contains=@Spell
 syn region cramOutput start=/^  [^$>]/ start=/^  $/ end=/\v.(\n\n*[^ ])\@=/me=s end=/^  [$>]/me=e-3 end=/^$/ fold containedin=cramBlock