[C++] Revert Buffer::mutable_data member to always inline

[asfimport]

If not, linkers must remember to define NDEBUG depending on whether Arrow was built in release mode or not

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #1756

_{Note: This issue was originally created as ARROW-2316. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 1756
#1756