[Java] Remove Netty dependency for ArrowBuf

[asfimport]

This is part of the first step of issue ARROW-4526.
In this step, we remove netty dependency for ArrowBuf, BufferAllocator and ReferenceManager.

In this issue, we remove the dependency for ArrowBuf.
The task for BufferAllocator and ReferenceManager will not start until ARROW-7329 is finished.

Reporter: Liya Fan / @liyafan82
Assignee: Liya Fan / @liyafan82

Related issues:

• [Java] Remove Netty references from ArrowBuf and move Allocator out of vector package (is a child of)

PRs and other links:

• GitHub Pull Request #6131

_{Note: This issue was originally created as ARROW-7505. Please see the migration documentation for further details.}

[asfimport]

Liya Fan / @liyafan82:
Fixed by #6131