sql: get rid of FuncDef function hash

Now it is possible to move all SQL builtin functions to
Tarantool's function hash. An existent FuncDef function
representation was replaced with func_sql_builtin class.
It has a sql-specific method :call and :finalize, while
port API call is not supported and protected with stubs.

This patch removes FuncDef hash and sql_function_create endpoint,
but doesn't introduce something instead. Therefore few affected
tests are disabled. A required functionality would be fixed in
the next patch.

Following tests using sql_create_function are broken now.
They are going to be fixed in the next commit:
 sql-tap/alias.test.lua sql-tap/check.test.lua
 sql-tap/func5.test.lua sql-tap/lua_sql.test.lua
 sql-tap/subquery.test.lua sql-tap/trigger9.test.lua
 sql/errinj.result sql/errinj.test.lua
 sql/func-recreate.test.lua

Part of #2200, #4113, #2233

@TarantoolBot document
Title: SQL builtins priveleges

All SQL built-ins are executed on SQL privilege level that is
undefined yet.