git migration: Remove support for file versions

Git does not support the ability to replace a token with a version
string during check-in. While it does have support for replacing a
token on clone, this is somewhat sub-optimal: the token is replaced
with the object hash, which is not particularly easy for human
consumption. What's more, in practice, the source file version was often
not terribly useful. Generally, when triaging bugs, the overall version
of Asterisk is far more useful than an individual SVN version of a file.
As a result, this patch removes Asterisk's support for showing source file
versions.

Specifically, it does the following:
* main/asterisk:
  - Refactor the file_version structure to reflect that it no longer
    tracks a version field.
  - Alter the "core show file version" CLI command such that it always
    reports the version of Asterisk. The file version is no longer
    available.

* main/manager: The Version key now always reports the Asterisk version.

* UPGRADE: Add notes for:
  - Modification to the ModuleCheck AMI Action.
  - Modification to the CLI "core show file version" command.

Change-Id: Ia932d3c64cd18a14a3c894109baa657ec0a85d28