Fix ERA5 without get_frequency

[bouweandela]

Remove the get_frequency function again and accept any frequency starting with 1hr as hourly.

I also noticed that the time shift applied in the function fix_hourly_time_coordinate assumes the time coordinate is in days, while in the tests and in the ERA5 data I have available locally/downloaded from ECWMF, it is in hours.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.42%. Comparing base (9dbf26c) to head (4b58227).

Files with missing lines	Patch %	Lines
esmvalcore/cmor/_fixes/native6/era5.py	90.90%	4 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##           era5_native6_monthly    #2855      +/-   ##
========================================================
- Coverage                 95.46%   95.42%   -0.04%     
========================================================
  Files                       260      260              
  Lines                     15527    15513      -14     
========================================================
- Hits                      14823    14804      -19     
- Misses                      704      709       +5     

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[flicj191]

Ah! Thank you! makes sense