Remove unnecessary "Find shell command" action, fix prompt typo and fix some actions

Remove because it is not necessary in Projectile since Projectile
already displays all files in Project for finding already. Also, it's
buggy for Projectile.

Fix prompt typo for helm-projectile-find-dir.

These Helm actions are fixed:

- Improve helm-projectile-switch-to-shell: make it also work with
  helm-projectile-find-file. Reuse helm-ff-switch-to-eshell.

- Fix "Helm Etags Select" action: now from helm-projectile-find-file, the action
  helm-etags-select can be invoked to list all available tags. User
  must generate a TAGS database and load into Emacs with visit-tag-table
  first. While in helm-etags session, pressing "C-c p f" switches back
  to helm-projectile-find-files.

- Fix "Eshell command on file(s)" action: now properly retrieve files to
  run commands on.