Remove suspicious error/warning suppressions from MTR

We have quite a few error/warning suppressions in MTR, both in mysql-test-run.pl and in mtr_warnings.sql. Some of them are difficult to explain, e.g.

 ("Got error [0-9]* when reading table"),

(in mtr_warnings.sql).

We need to try to remove them, run tests, see if anything is affected, possibly fix if something is affected while it shouldn't be, and then push (or decide that we'd better keep them as is).