Remove redundant check in formula_files and cask_files
Remove redundant check in formula_files and cask_files

Formula_files consists of every non-recursive child of formula_dir,
for which formula_file? evaluates to true. formula_file? checks if the file
is a child of formula_dir, which it is by definition. It turns out that by
removing the check, the time used for 'brew search' decreased from 800 ms to
700 ms, noticably faster during tab completion. The same happens with
cask_files and cask_file?

EricFromCanada/brew,sjackman/homebrew,sjackman/homebrew,Linuxbrew/brew,EricFromCanada/brew,nandub/brew,Linuxbrew/brew,Linuxbrew/brew,nandub/brew,JCount/brew,konqui/brew,konqui/brew,vitorgalvao/brew,reitermarkus/brew,mahori/brew,reitermarkus/brew,MikeMcQuaid/brew,vitorgalvao/brew,konqui/brew,Homebrew/brew,Linuxbrew/brew,JCount/brew,vitorgalvao/brew,mahori/brew,claui/brew,JCount/brew,nandub/brew,konqui/brew,vitorgalvao/brew,reitermarkus/brew,MikeMcQuaid/brew,mahori/brew,nandub/brew,Homebrew/brew,JCount/brew,EricFromCanada/brew,sjackman/homebrew,claui/brew,claui/brew,Homebrew/brew,EricFromCanada/brew,claui/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,sjackman/homebrew,reitermarkus/brew,Homebrew/brew,mahori/brew