revert proxy statement cache & synchronized register method.

[cherrylzhao]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 7026

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.0008%) to 71.134%

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Files with Coverage Reduction	New Missed Lines	%
sharding-proxy/sharding-proxy-frontend/sharding-proxy-frontend-mysql/src/main/java/org/apache/shardingsphere/shardingproxy/frontend/mysql/command/query/binary/close/MySQLComStmtCloseExecutor.java	1	0.0%

Totals
Change from base Build 7024:	0.0008%
Covered Lines:	11299
Relevant Lines:	15884

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💛 - Coveralls