Fix AsPartialPerm method for transformations

[james-d-mitchell]

Please make sure that this pull request:

• is submitted to the correct branch (the stable branch is only for bugfixes)
• contains an accurate description of changes for the release notes below
• provides new tests or relies on existing ones
• correctly refers to other issues and related pull requests

Tick all what applies to this pull request

• Adds new features
• Improves and extends functionality
• Fixes bugs that could lead to crashes
• Fixes bugs that could lead to incorrect results
• Fixes bugs that could lead to break loops

Write below the description of changes (for the release notes)

The previous method was written when transformations had a fixed degree,
and was incorrect when applied to a degree-less transformation (i.e. all
transformations in GAP since GAP 4.7).

The method, related tests and doc, for AsPartialPerm of a transformation
(and no further args) were removed. It is not very clear what such a
method should do.

[codecov-io]

Current coverage is 49.57% (diff: 0.00%)

Merging #1036 into master will increase coverage by 0.01%

@@             master      #1036   diff @@
==========================================
  Files           424        424          
  Lines        223277     223264    -13   
  Methods        3430       3430          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits         110656     110689    +33   
+ Misses       112621     112575    -46   
  Partials          0          0          

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[fingolfin]

This seems sensible to me.