ddl: remove wait time after canceling a job

[zimulala]

What problem does this PR solve?

When multiple TiDBs are running bootstrap at the same time, the following information will be displayed:

...
2019/03/07 13:52:03.747 ddl_worker.go:543: [info] [ddl-worker 1, tp general] the DDL job is normal to cancel because [schema:1007]Can't create database 'test'; database exists
2019/03/07 13:52:03.747 session.go:951: [warning] con:0 compile error:
[schema:1146]Table 'mysql.global_variables' doesn't exist
select HIGH_PRIORITY variable_name, variable_value from mysql.global_variables where variable_name in ('tidb_ddl_error_count_limit')
2019/03/07 13:52:03.747 ddl_worker.go:550: [error] [ddl-worker 1, tp general] load ddl global variable error: [schema:1146]Table 'mysql.global_variables' doesn't exist
...
ArgLen:1, start time: 2019-03-07 13:52:02.701 +0000 UTC, Err:[schema:1007]Can't create database 'test'; database exists, ErrCount:1, SnapshotVersion:0
2019/03/07 13:52:03.753 ddl_worker.go:415: [info] [ddl-worker 1, tp general] run DDL job error, sleeps a while:1s then retries it.
...

What is changed and how it works?

When the job is canceled, we needn't load DDL global variables or sleep for a while.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

[winkyao]

LGTM

[zimulala]

/run-all-tests

[crazycs520]

LGTM

[zimulala]

/run-unit-test

[codecov]

Codecov Report

Merging #9648 into master will increase coverage by 0.0187%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##             master      #9648        +/-   ##
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+ Coverage   67.2198%   67.2386%   +0.0187%     
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  Files           381        381                
  Lines         79835      79835                
================================================
+ Hits          53665      53680        +15     
+ Misses        21381      21372         -9     
+ Partials       4789       4783         -6

[xiekeyi98]

LGTM