Deprecating Legacy Timer

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

MetOffice/lfric_apps#80, #201, Metoffice/lfric_apps#152

### Brief description

Brief description

Once the timing system is reworked to offer full Vernier functionality, as part of issues MetOffice/lfric_apps#68 and #192 and respective PRs MetOffice/lfric_apps#80 and #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.

The lines
```
ifdef USE_LEGACY_TIMER
export PRE_PROCESS_MACROS += LEGACY_TIMER
endif
```
and
```
# Check that only one profiler is requested
ifneq ($(and $(findstring LEGACY_TIMER, $(PRE_PROCESS_MACROS)), \
$(findstring VERNIER, $(PRE_PROCESS_MACROS))), )
$(error Multiple profilers specified, limit choice to single profiler.)
endif

# Check that TIMING ON has been set if any profiler requested.
ifneq ($(or $(findstring LEGACY_TIMER,$(PRE_PROCESS_MACROS)), \
$(findstring VERNIER, $(PRE_PROCESS_MACROS))), )
ifndef USE_TIMING_WRAPPER
$(error Conflicting options: Profiler requested with Timing disabled.)
endif
endif
```
in lfric.mk can be removed or significantly simplified.

Read lfric.mk and review the linked timing-system issues and pull requests, especially MetOffice/lfric_apps#68, #192, #80, and #201. After Vernier functionality and post-processing are available, remove or simplify the USE_LEGACY_TIMER and profiler-validation logic, with the legacy timer no longer supported.

Written by the indexing model from the issue text.