[SPARK-6555] [SQL] Overrides equals() and hashCode() for MetastoreRelation

Also removes temporary workarounds made in #5183 and #5251.

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Author: Cheng Lian <lian@databricks.com>

Closes #5289 from liancheng/spark-6555 and squashes the following commits:

d0095ac [Cheng Lian] Removes unused imports
cfafeeb [Cheng Lian] Removes outdated comment
75a2746 [Cheng Lian] Overrides equals() and hashCode() for MetastoreRelation