Drop doc/main.txt

This is no longer needed since we don't use the internal online help
system anymore.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

doc/Makefile.am

@@ -30,7 +30,6 @@ dist_doc_DATA = \
 	goto.txt \
 	import.txt \
 	intro.txt \
-	main.txt \
 	manual.txt \
 	other.txt \
 	pipe.txt \

doc/main.txt

@@ -1,20 +0,0 @@
-       Welcome to Calcurse. This is the main help screen.
-=========================================================
-
-Moving around:  Press '^P' or '^N' to scroll text upward or downward
-                inside help screens, if necessary.
-
-    Exit help:  When finished, press 'q' to exit help and go back to
-                the main Calcurse screen.
-
-   Help topic:  At the bottom of this screen you can see a panel with
-                different fields, represented by a letter and a short
-                title. This panel contains all the available actions
-                you can perform when using Calcurse.
-                By pressing one of the letters appearing in this
-                panel, you will be shown a short description of the
-                corresponding action. At the top right side of the
-                description screen are indicated the user-defined key
-                bindings that lead to the action.
-
-      Credits:  Press '@' for credits.