[Xamarin.Android.Build.Tasks] remove unused Inputs/Outputs

[jonathanpeppers]

While reviewing our MSBuild targets, I noticed three targets that
declared Inputs and Outputs, but did not actually update any
files:

• _CollectConfigFiles
• _CollectMdbFiles
• _CollectPdbFiles

This means that MSBuild was unnecessarily looking at files to see if
they exist, and might be comparing timestamps as well. The targets
would also always run, so it doesn't matter to have Inputs and
Outputs in these targets at all!

I basically removed these Inputs and Outputs, and saw a
performance gain with just doing that. I also saw performance gains to
targets that depend on these three targets. Since Inputs and
Outputs timing is internal to MSBuild, I am not sure where the
actual time is being tallied here.

Before:
    1 ms  _CopyConfigFiles                           1 calls
    3 ms  _CopyMdbFiles                              2 calls
    3 ms  _CollectMdbFiles                           1 calls
    4 ms  _CopyPdbFiles                              1 calls
    5 ms  _CollectPdbFiles                           1 calls
    5 ms  _CollectConfigFiles                        1 calls
   20 ms  _ConvertPdbFiles                           1 calls

A total of ~46ms

After:
    0 ms  _ConvertPdbFiles                           1 calls
    0 ms  _CopyPdbFiles                              1 calls
    1 ms  _CollectMdbFiles                           1 calls
    1 ms  _CopyMdbFiles                              2 calls
    1 ms  _CopyConfigFiles                           1 calls
    2 ms  _CollectConfigFiles                        1 calls
    5 ms  _CollectPdbFiles                           1 calls

A total of ~10ms

An overall improvement of ~36ms.

This was the samples/HelloWorld project in this repo, a build with
no changes. I suspect the improvement would be even better for larger
projects.

[jonathanpeppers]

Build logs for this change: logs.zip

[dellis1972]

@jonathanpeppers looks like I was a bit overzealous with the Inputs and Outputs then :) This looks good.