* org-agenda.el (org-agenda-mode-map): Use ?= for filtering by
regexp and ?| for removing all filters.
(org-agenda-filter-remove-all): New command.
(org-agenda-filter-show-all-re): Rename from
`org-agenda-filter-show-all-regexp'.
(org-agenda-filter-by-regexp): Call
`org-agenda-filter-show-all-re'.