Fixed BUG#18949: Test case sp-goto is disabled

Removed sp-goto.test, sp-goto.result and all (disabled) GOTO code.
  Also removed some related code that's not needed any more (no possible
  unresolved label references any more, so no need to check for them).
  NB: Keeping the ER_SP_GOTO_IN_HNDLR in errmsg.txt; it might become useful
      in the future, and removing it (and thus re-enumerating error codes)
      might upset things. (Anything referring to explicit error codes.)


BitKeeper/deleted/.del-sp-goto.result~f343103c63f64b7a:
  Delete: mysql-test/r/sp-goto.result
BitKeeper/deleted/.del-sp-goto.test~5054d3f729692d3d:
  Delete: mysql-test/t/sp-goto.test
mysql-test/t/disabled.def:
  sp-goto.test no longer exists.
sql/lex.h:
  Removed (disabled) GOTO definitions.
sql/sp_head.cc:
  Removed sp_head::check_backpatch() and simplified sp_head::backpatch().
  Without GOTO, unresolved label references are not possible, so no need
  to check for them.
sql/sp_head.h:
  Removed sp_head::check_backpatch(). (Not needed with no GOTO)
sql/sp_pcontext.cc:
  SP_LAB_GOTO was renamed to SP_LAB_IMPL
sql/sp_pcontext.h:
  Removed SP_LAB_REF (no longer needed) and renamed SP_LAB_GOTO
  to SP_LAB_IMPL, since it's only used for implicit labels now.
sql/sql_yacc.yy:
  Removed GOTO symbols and (disabled) code, and the no longer needed
  sp_head::check_backpatch() calls.

sql/sp_pcontext.cc

@@ -241,7 +241,7 @@ sp_pcontext::push_label(char *name, uint ip)
   {
     lab->name= name;
     lab->ip= ip;
-    lab->type= SP_LAB_GOTO;
+    lab->type= SP_LAB_IMPL;
     lab->ctx= this;
     m_label.push_front(lab);
   }