vim: be more careful when removing the documentation

The current VIM_REMOVE_DOCS hook removes all .txt files from
/usr/share/vim. Unfortunately, this also removes the rgb.txt file,
which is needed at runtime for vim, as reported in bug #9466.

This commit changes VIM_REMOVE_DOCS to remove only
/usr/share/vim/vim*/doc/. Size-wise, it's equivalent because:

 - We are no longer removing a few README.txt in other directories,
   taking more space.

 - We are now removing the /usr/share/vim/vim*/doc/ folder entirely,
   which contained a few files not named *.txt

So overall, the size of /usr/share/vim/ before and after this patch is
still 11MB.

Fixes bug #9466.

Reported-by: Mateusz Furdyna <sir.ferdek@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
(cherry picked from commit f7a07f42f7)

package/vim/vim.mk

@@ -61,7 +61,7 @@ define VIM_INSTALL_RUNTIME_CMDS
 endef
 
 define VIM_REMOVE_DOCS
-	find $(TARGET_DIR)/usr/share/vim -type f -name "*.txt" -delete
+	$(RM) -rf $(TARGET_DIR)/usr/share/vim/vim*/doc/
 endef
 
 # Avoid oopses with vipw/vigr, lack of $EDITOR and 'vi' command expectation