patch 8.1.1901: the +insert_expand feature is not always available

Problem:    The +insert_expand feature is not always available.
Solution:   Graduate the +insert_expand feature.
diff --git a/src/charset.c b/src/charset.c
index 50ca617..d47ccfd 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -314,8 +314,6 @@
     }
 }
 
-#if defined(FEAT_EVAL) || defined(FEAT_TITLE) || defined(FEAT_INS_EXPAND) \
-	|| defined(PROTO)
 /*
  * Translate a string into allocated memory, replacing special chars with
  * printable chars.  Returns NULL when out of memory.
@@ -382,9 +380,7 @@
     }
     return res;
 }
-#endif
 
-#if defined(FEAT_SYN_HL) || defined(FEAT_INS_EXPAND) || defined(PROTO)
 /*
  * Convert the string "str[orglen]" to do ignore-case comparing.  Uses the
  * current locale.
@@ -495,7 +491,6 @@
 	return (char_u *)ga.ga_data;
     return buf;
 }
-#endif
 
 /*
  * Catch 22: g_chartab[] can't be initialized before the options are
@@ -2015,6 +2010,7 @@
 
 /*
  * backslash_halve() plus save the result in allocated memory.
+ * However, returns "p" when out of memory.
  */
     char_u *
 backslash_halve_save(char_u *p)