Remove panics from reportMissingVariationDependency

The code that has been removed triggers in many cases when it should
not, e.g. an unknown module on the bootclasspath. The code was
originally added to try and ensure that if the attempt to add useful
error messages failed it would help debug the issue. However, it ended
up just creating more failures.

If this does become an issue then this will be revisited.

Bug: 177892522
Test: Add an unknown module to PRODUCT_BOOT_JARS
      m SOONG_ALLOW_MISSING_DEPENDENCIES=true
Change-Id: I08dbd4b29d301f3f1f78a0fb396687062b47deb8