remove disable_char_avail_for_testing()

test_cursor_func.vim hangs at the call to
disable_char_avail_for_testing(). The test does not actually need this
function (and it correctly fails if the fix from 7.4.1300 is reverted).
Given that disable_char_avail_for_testing is a gigantic hack, if we can
avoid it let's do so.

src/nvim/version.c

@@ -377,7 +377,7 @@ static int included_patches[] = {
   // 1303 NA
   // 1302 NA
   // 1301 NA
-  1300,
+  // 1300 NA
   // 1299 NA
   // 1298 NA
   // 1297 NA