remove virtual package all-rocks

> There's also now an automatic all-<plugin> target which is the
> last segment of the plugin path. all-rocks.mk can be removed
> and make all-luarocks MXE_PLUGIN_DIRS=plugins/luarocks ...
> will work.

See #1017

plugins/luarocks/README.md

@@ -73,7 +73,7 @@ installer and Lua library (it installs Lua files to
 
 Build all rocks:
 ```
-$ make all-rocks MXE_PLUGIN_DIRS=plugins/luarocks MXE_TARGETS='i686-w64-mingw32.shared x86_64-w64-mingw32.shared'
+$ make all-luarocks MXE_PLUGIN_DIRS=plugins/luarocks MXE_TARGETS='i686-w64-mingw32.shared x86_64-w64-mingw32.shared'
 ```
 
 Run tests (requires wine):

plugins/luarocks/all-rocks.mk

@@ -1,7 +0,0 @@
-# This file is part of MXE.
-# See index.html for further information.
-
-PKG             := all-rocks
-$(PKG)_WEBSITE  := https://luarocks.org/
-$(PKG)_OWNER    := https://github.com/starius
-$(PKG)_DEPS     := lpeg luasocket llthreads2 lua-rapidjson