Remove ScoverageXMLMerger class and its test.

This class was added in 9175228 and was used only in small period of time between release 0.99.11 and 1.0.0 (from 83d9996/scalac-scoverage-plugin/src/main/scala/scoverage/IOUtils.scala to 2bfcbbf/scalac-scoverage-plugin/src/main/scala/scoverage/IOUtils.scala).

The class responsible for aggregating reports is now [CoverageAggregator](https://github.com/scoverage/scalac-scoverage-plugin/blob/v1.3.0-RC2/scalac-scoverage-plugin/src/main/scala/scoverage/report/CoverageAggregator.scala).

Author: gslowikowski