dropped "system" flag for ExApps

[bigcat88]

We are making ExApps more like usual PHP Apps for Nextcloud.

Now any ExApp is allowed to impersonate any user, no need "system" flag for this.

Reference: nextcloud/app_api#323

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.72%. Comparing base (4f2bbf6) to head (f8a9a3a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
- Coverage   94.74%   94.72%   -0.02%     
==========================================
  Files          47       47              
  Lines        5403     5403              
==========================================
- Hits         5119     5118       -1     
- Misses        284      285       +1     

Files	Coverage Δ
nc_py_api/_session.py	96.44% <ø> (ø)
nc_py_api/_version.py	100.00% <100.00%> (ø)
nc_py_api/nextcloud.py	96.14% <ø> (ø)
nc_py_api/apps.py	99.23% <0.00%> (-0.77%)	⬇️