deps: remove direct declaration of google-auth-library-oauth2-http (#184)

Rather than having a direct managed dependency for
google-auth-library-oauth2-http use the version provided by
google-cloud-shared-dependencies.

Author: BenWhitehead

pom.xml

@@ -180,11 +180,6 @@
           </exclusion>
         </exclusions>
       </dependency>
-      <dependency>
-        <groupId>com.google.auth</groupId>
-        <artifactId>google-auth-library-oauth2-http</artifactId>
-        <version>0.20.0</version>
-      </dependency>
       <dependency>
         <groupId>com.google.oauth-client</groupId>
         <artifactId>google-oauth-client</artifactId>