Remove a couple other vestigial yylex() declarations.

These were workarounds for a long-gone flex bug; all supported versions
of flex emit an extern declaration as expected.

src/backend/utils/misc/guc-file.l

@@ -47,9 +47,6 @@ static sigjmp_buf *GUC_flex_fatal_jmp;
 
 static void FreeConfigVariable(ConfigVariable *item);
 
-/* flex fails to supply a prototype for yylex, so provide one */
-int GUC_yylex(void);
-
 static int GUC_flex_fatal(const char *msg);
 static char *GUC_scanstr(const char *s);
 

src/bin/psql/psqlscan.l

@@ -114,8 +114,6 @@ static int	backtick_start_offset;
 #define LEXRES_OK			3	/* OK completion of backslash argument */
 
 
-int	yylex(void);
-
 static void evaluate_backtick(void);
 static void push_new_buffer(const char *newstr, const char *varname);
 static void pop_buffer_stack(PsqlScanState state);