simplify

[cloud-fan]

I tried to address my own comments and add the key and value label, but had no luck.

This PR just does 2 simplifications:

1. reuse the quotedIdentifier parser rule instead of stripping backticks by our own
2. remove the value label as we can't use it anyway.

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This change is 

[cloud-fan]

cc @maropu

[maropu]

Thanks! I checked that SparkSqlParserSuite passed and merged it.