Remove asc signatures on checksums in MavenPublisher

[l46kok]

It is neither necessary, nor correct to include asc signature files with checksums. See https://central.sonatype.org/publish/requirements/#sign-files-with-gpgpgp

Previously, uploading these files were simply a no-op. But with Sonatype's recent migration to Central Publisher Portal -- it will result in a 500 error if these are uploaded:

Exception in thread "main" java.util.concurrent.ExecutionException: java.io.IOException: Unable to upload https://central.sonatype.com/repository/maven-snapshots/dev/cel/protobuf/0.10.1-TEST-SNAPSHOT/protobuf-0.10.1-TEST-SNAPSHOT.pom.md5.asc (500)
        at java.base/java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:395)
        at java.base/java.util.concurrent.CompletableFuture.get(CompletableFuture.java:2028)
        at com.github.bazelbuild.rules_jvm_external.maven.MavenPublisher.main(MavenPublisher.java:131)
Caused by: java.io.IOException: Unable to upload https://central.sonatype.com/repository/maven-snapshots/dev/cel/protobuf/0.10.1-TEST-SNAPSHOT/protobuf-0.10.1-TEST-SNAPSHOT.pom.md5.asc (500)
        at com.github.bazelbuild.rules_jvm_external.maven.MavenPublisher.lambda$httpUpload$3(MavenPublisher.java:337)
        at com.github.bazelbuild.rules_jvm_external.maven.MavenPublisher.lambda$upload$0(MavenPublisher.java:283)
        at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
        at java.base/java.lang.Thread.run(Thread.java:829)

(Ideally, maven central's server validation should be fixed to not throw a 500 error, but that's a separate issue)

[shs96c]

If it'll prevent 500 errors occurring on the major hub of OSS java development, it seems like a reasonable thing to do to drop these. Thank you for the PR!