Remove java.toolchain from build, fix Win arm64

After #73, running `.\gradlew.bat build` on arm64 Windows would fail
with "WindowsRegistry is not supported on this operating system."
Removing the `java.toolchain` block from strcalc/build.gradle.kts allows
it to succeed, as Gradle no longer tries to query the Windows registry
to discover the Java toolchain. It will just pick up the toolchain from
the environment.

Running `.\gradlew.bat -q javaToolchains` will still produce the
failure, however.

See:
- https://docs.gradle.org/current/userguide/toolchains.html
- gradle/native-platform: WindowsRegistry is not supported on this
  operating system. #274
  gradle/native-platform#274 (comment)
- gradle/gradle: Support ARM64 Windows #21703
  gradle/gradle#21703

strcalc/build.gradle.kts

@@ -39,13 +39,6 @@ dependencies {
     antJUnit(libs.antJunit)
 }
 
-// Apply a specific Java toolchain to ease working on different environments.
-java {
-    toolchain {
-        languageVersion.set(JavaLanguageVersion.of(21))
-    }
-}
-
 jacoco {
     toolVersion = "0.8.11"
 }