patch 8.1.1034: too many #ifdefs

Problem:    Too many #ifdefs.
Solution:   Merge FEAT_MOUSE_SGR into FEAT_MOUSE_XTERM / FEAT_MOUSE_TTY.
diff --git a/src/feature.h b/src/feature.h
index 284c8e0..e16fc62 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -1026,9 +1026,6 @@
 # ifdef FEAT_BIG
 #  define FEAT_MOUSE_URXVT
 # endif
-# ifdef FEAT_BIG
-#  define FEAT_MOUSE_SGR
-# endif
 # if defined(FEAT_NORMAL) && defined(MSWIN)
 #  define DOS_MOUSE
 # endif
@@ -1057,11 +1054,6 @@
 # define FEAT_MOUSE_XTERM
 #endif
 
-/* sgr is a small variation of mouse_xterm, and shares its code */
-#if defined(FEAT_MOUSE_SGR) && !defined(FEAT_MOUSE_XTERM)
-# define FEAT_MOUSE_XTERM
-#endif
-
 /* Define FEAT_MOUSE when any of the above is defined or FEAT_GUI. */
 #if !defined(FEAT_MOUSE_TTY) \
 	&& (defined(FEAT_MOUSE_XTERM) \
@@ -1072,8 +1064,7 @@
 	    || defined(FEAT_MOUSE_JSB) \
 	    || defined(FEAT_MOUSE_PTERM) \
 	    || defined(FEAT_SYSMOUSE) \
-	    || defined(FEAT_MOUSE_URXVT) \
-	    || defined(FEAT_MOUSE_SGR))
+	    || defined(FEAT_MOUSE_URXVT))
 # define FEAT_MOUSE_TTY		/* include non-GUI mouse support */
 #endif
 #if !defined(FEAT_MOUSE) && (defined(FEAT_MOUSE_TTY) || defined(FEAT_GUI))