Fix documentation of field `soong.apex.apexBundleProperties.Apex_name`.

Remove the mention to the (APEX) manifest (`apex_manifest.json`) from
this field's documentation, as `Apex_name` is neither read from nor
written to that file.

Test: n/a
Change-Id: Ia0dfac4f35f1ea697f379bcb19dd11af8705a0a0