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/os_unix.c b/src/os_unix.c
index 0d9f69e..b398297 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -3596,7 +3596,6 @@
     }
 # endif
 
-# ifdef FEAT_MOUSE_SGR
     if (ttym_flags == TTYM_SGR)
     {
 	/* SGR mode supports columns above 223 */
@@ -3606,7 +3605,6 @@
 		   : IF_EB("\033[?1006l", ESC_STR "[?1006l")));
 	mouse_ison = on;
     }
-# endif
 
 # ifdef FEAT_BEVAL_TERM
     if (bevalterm_ison != (p_bevalterm && on))
@@ -3864,11 +3862,10 @@
     else
 	del_mouse_termcode(KS_URXVT_MOUSE);
 # endif
-# ifdef FEAT_MOUSE_SGR
     if (use_xterm_mouse() == 4
-#  ifdef FEAT_GUI
+# ifdef FEAT_GUI
 	    && !gui.in_use
-#  endif
+# endif
 	    )
     {
 	set_mouse_termcode(KS_SGR_MOUSE, (char_u *)(term_is_8bit(T_NAME)
@@ -3890,7 +3887,6 @@
 	del_mouse_termcode(KS_SGR_MOUSE);
 	del_mouse_termcode(KS_SGR_MOUSE_RELEASE);
     }
-# endif
 }
 #endif