Revert "allow model names starting with underscore _"

This reverts commit df411ad1c9.

src/frontend/inpcom.c

@@ -1821,10 +1821,6 @@ is_a_modelname(const char *s)
     if (isalpha_c(s[0]))
         return TRUE;
 
-    /* people use model names starting with '_' */
-    if (s[0] == '_')
-        return TRUE;
-
     /* e.g. 1N4002, but do not accept floats (for example 1E2) */
     if (isdigit_c(s[0]) && isalpha_c(s[1]) && isdigit_c(s[2]) && toupper_c(s[1]) != 'E')
         return TRUE;