🔥 remove clippy.toml with single msrv field, let clippy use rust-version in Cargo.toml instead

[ttytm]

With my last update of the msrv I missed to update the msrv in clippy.toml.
The deviating versions are now causing a warning when running clippy.

Since the clippy.toml file only contains the msrv and specifying the msrv as rust-version field in Cargo.toml is supported for a while now, it seems simpler and sufficient to let clippy use this single field.

[github-actions]

Filename                              Stmts    Miss  Cover    Missing
----------------------------------  -------  ------  -------  ------------------------------------------------------------
src/main.rs                              13      13  0.00%    26-43
src/modules/display/border.rs            47      47  0.00%    17-106
src/modules/display/current.rs           59      59  0.00%    37-222
src/modules/display/day.rs               69      69  0.00%    30-162
src/modules/display/graph.rs            169     164  2.96%    65-368
src/modules/display/gui_config.rs        15      12  20.00%   46-61
src/modules/display/historical.rs        71      71  0.00%    36-198
src/modules/display/hourly.rs           158     158  0.00%    40-397
src/modules/display/product.rs           21      21  0.00%    24-64
src/modules/display/utils.rs             38      38  0.00%    7-76
src/modules/display/weathercode.rs       31      31  0.00%    11-41
src/modules/display/week.rs              44      44  0.00%    31-123
src/modules/display/wind.rs              21      21  0.00%    17-42
src/modules/api.rs                       24      13  45.83%   26-29, 38, 49-88
src/modules/args.rs                       4       4  0.00%    91-95
src/modules/config.rs                    31      25  19.35%   52-97
src/modules/forecast.rs                  20       6  70.00%   7-8, 24-25, 27-28
src/modules/localization.rs             105      41  60.95%   162-203, 230-265
src/modules/location.rs                  53      28  47.17%   25-29, 59-63, 75, 89, 95-104, 109, 118-126, 131-132, 142-143
src/modules/params.rs                    52      52  0.00%    23-133
src/modules/units.rs                     13       0  100.00%
src/modules/weather.rs                   28      28  0.00%    83-154
TOTAL                                  1086     945  12.98%

[github-actions]

Test Results

9 tests  ±0   9 ✅ ±0   1s ⏱️ ±0s
1 suites ±0   0 💤 ±0 
1 files   ±0   0 ❌ ±0 

Results for commit 49e967b. ± Comparison against base commit f5f71bc.