test: Remove the io.sentry.systemtest plugin and sentry-system-test-support

With the runner gone, nothing applies the io.sentry.systemtest
convention plugin or depends on sentry-system-test-support, which held
the HTTP and GraphQL clients and the TestHelper the system tests used.
The envelope matchers it delegated to stay in sentry-test-support.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: runningcode

build-logic/src/main/kotlin/io.sentry.systemtest.gradle.kts

@@ -87,8 +87,7 @@ apiValidation {
             "test-app-size",
             "sentry-samples-netflix-dgs",
             "sentry-samples-console-otlp",
-            "sentry-test-support",
-            "sentry-system-test-support"
+            "sentry-test-support"
         )
     )
 }
@@ -126,7 +125,7 @@ subprojects {
         }
     }
 
-    if (!this.name.contains("sample") && !this.name.contains("integration-tests") && this.name != "sentry-system-test-support" && this.name != "sentry-test-support") {
+    if (!this.name.contains("sample") && !this.name.contains("integration-tests") && this.name != "sentry-test-support") {
         apply<DistributionPlugin>()
         apply<com.vanniktech.maven.publish.MavenPublishPlugin>()