patch 8.1.1251: no test for completion of mapping keys

Problem:    No test for completion of mapping keys.
Solution:   Add a test.  Also clean up the code.
diff --git a/src/proto/term.pro b/src/proto/term.pro
index 1b8ab5f..136a969 100644
--- a/src/proto/term.pro
+++ b/src/proto/term.pro
@@ -74,7 +74,7 @@
 int find_term_bykeys(char_u *src);
 void show_termcodes(void);
 int show_one_termcode(char_u *name, char_u *code, int printit);
-char_u *translate_mapping(char_u *str, int expmap);
+char_u *translate_mapping(char_u *str);
 void update_tcap(int attr);
 void swap_tcap(void);
 guicolor_T gui_get_color_cmn(char_u *name);