Remove relative-source-error test

The io library now delegates to swoval for io which generally handles
relative sources correctly (by converting them to absolute paths
internally). This test started failing after the io bump because of
this. It seemed to me that relative sources not working was not a
feature so I just removed the test.

sbt/src/sbt-test/source-dependencies/relative-source-error/changes/absolute.sbt

@@ -1 +0,0 @@
-scalaSource in Compile := baseDirectory.value / "src"

sbt/src/sbt-test/source-dependencies/relative-source-error/changes/relative.sbt

@@ -1 +0,0 @@
-scalaSource in Compile := file("src")

sbt/src/sbt-test/source-dependencies/relative-source-error/src/A.scala

@@ -1 +0,0 @@
-object A

sbt/src/sbt-test/source-dependencies/relative-source-error/test

@@ -1,7 +0,0 @@
-$ copy-file changes/relative.sbt build.sbt
-> reload
--> compile
-
-$ copy-file changes/absolute.sbt build.sbt
-> reload
-> compile