[Java] Combine MapVector and NonNullableMapVector Classes

[asfimport]

NonNullableMapVector class can be merged into MapVector and removed as part of removing the non nullable vectors.

Reporter: Bryan Cutler / @BryanCutler
Assignee: Bryan Cutler / @BryanCutler

Related issues:

• [Java] Rename MapVector to StructVector (blocks)
• [Java] Remove non-nullable vectors in new vector class hierarchy (is related to)

PRs and other links:

• GitHub Pull Request #1371

_{Note: This issue was originally created as ARROW-1866. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
Transitioning issue from Resolved to Closed to based on resolution field value.