refactor(plugins): remove old generated completion files

This removes .gitignore plugin files and adds a force-rm of the old completion file,
which ignores any errors if the file doesn't exist.

plugins/rustup/rustup.plugin.zsh

@@ -1,4 +1,7 @@
 if (( $+commands[rustup] )); then
+  # remove old generated completion file
+  command rm -f "${0:A:h}/_rustup"
+
   ver="$(rustup --version 2>/dev/null)"
   ver_file="$ZSH_CACHE_DIR/rustup_version"
   comp_file="$ZSH_CACHE_DIR/completions/_rustup"