Remove hard-coded versions from Eclipse plugin

This is consistent with the versions for the other modules.

The release process updates the versions and pushes a tagged
commit, but the version in the main branch is always
'HEAD-SNAPSHOT'.

Follow-up to #744 (comment)

PiperOrigin-RevId: 571607356

eclipse_plugin/META-INF/MANIFEST.MF

@@ -3,7 +3,7 @@ Bundle-ManifestVersion: 2
 Bundle-Name: google-java-format
 Bundle-SymbolicName: google-java-format-eclipse-plugin;singleton:=true
 Bundle-Vendor: Google
-Bundle-Version: 1.13.0
+Bundle-Version: HEAD-SNAPSHOT
 Bundle-RequiredExecutionEnvironment: JavaSE-11
 Require-Bundle: org.eclipse.jdt.core;bundle-version="3.10.0",
  org.eclipse.jface,

eclipse_plugin/pom.xml

@@ -22,7 +22,7 @@
   <groupId>com.google.googlejavaformat</groupId>
   <artifactId>google-java-format-eclipse-plugin</artifactId>
   <packaging>eclipse-plugin</packaging>
-  <version>1.13.0</version>
+  <version>HEAD-SNAPSHOT</version>
 
   <name>Google Java Format Plugin for Eclipse 4.5+</name>