Remove sbt 1.5.x tests due to sbt-sourcegraph 0.4.4 incompatibility

sbt-sourcegraph 0.4.4 (built with sbt 1.11.1) is incompatible with
sbt 1.5.2, causing tests to hang indefinitely. sbt 1.10.x tests remain.

Author: jupblb

tests/buildTools/src/test/scala/tests/SbtBuildToolSuite.scala

@@ -1,7 +1,5 @@
 package tests
 
-import tests.Tool.SBT15
-
 abstract class SbtBuildToolSuite(sbt: Tool.SBT) extends BaseBuildToolSuite {
 
   import Tool._
@@ -62,5 +60,4 @@ abstract class SbtBuildToolSuite(sbt: Tool.SBT) extends BaseBuildToolSuite {
 
 import Tool._
 
-class Sbt_15_BuildToolSuite extends SbtBuildToolSuite(SBT15)
 class Sbt_110_BuildToolSuite extends SbtBuildToolSuite(SBT110)

tests/buildTools/src/test/scala/tests/Tool.scala

@@ -46,7 +46,6 @@ object Tool {
   sealed abstract class SBT(version: String, support: JVMSupport)
       extends Tool("sbt", version, support)
   // See https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html#build-tool-compatibility-table
-  case object SBT15 extends SBT("1.5.2", atMostJava(17))
   case object SBT110 extends SBT("1.10.0", noRestrictions)
 
   sealed abstract class Scala(version: String, support: JVMSupport)