Revert "Add PKGBUILD template"

Revert change of syntax/PKGBUILD.vim in commit
6fef11d8149190f164c5331f4bee401a26808607 "Add PKGBUILD template"

Make syntax/PKGBUILD.vim the same as
https://projects.archlinux.org/pacman.git/tree/contrib/PKGBUILD.vim
Refer to https://github.com/Firef0x/PKGBUILD.vim/issues/9