Deprecate legacy timer

### Are there any linked Issues or Pull Requests?

#80, MetOffice/lfric_core#201

### Brief description

Once the timing system is reworked to offer full Vernier functionality, as part of issues #68 and MetOffice/lfric_core#192 and respective PRs #80 and MetOffice/lfric_core#201, post-processing options for Vernier outputs can be added and eventually the legacy timer deprecated.

As part of this, the build system changes involving pre-processing macros can also be simplified.

### Further details of the issue.

_No response_

Start by reading linked issues #68 and MetOffice/lfric_core#192 and their pull requests #80 and MetOffice/lfric_core#201 to understand the Vernier timing rework. Locate the legacy timer and preprocessing macro changes described there. Done means the legacy timer is deprecated and the related build-system preprocessing options are simplified after Vernier post-processing is available.

Written by the indexing model from the issue text.