cleanup: remove deprecated ConnectionProperty class (googleapis#3701)

Removes the deprecated ConnectionOptions$ConnectionProperty class. This class is
defined in the com.google.cloud.spanner.connection package, which is marked as an
internal API package. In addition, the enclosing ConnectionProperties class is also
marked as `@InternalApi`, which makes it acceptable to remove this class without
doing a major version bump.

Author: olavloite

google-cloud-spanner/clirr-ignored-differences.xml

@@ -850,4 +850,16 @@
     <className>com/google/cloud/spanner/connection/Connection</className>
     <method>java.lang.String getDefaultSequenceKind()</method>
   </difference>
+
+  <!-- Removed ConnectionOptions$ConnectionProperty in favor of the more generic ConnectionProperty class. -->
+  <difference>
+    <differenceType>8001</differenceType>
+    <className>com/google/cloud/spanner/connection/ConnectionOptions$ConnectionProperty</className>
+  </difference>
+  <difference>
+    <differenceType>6001</differenceType>
+    <className>com/google/cloud/spanner/connection/ConnectionOptions</className>
+    <field>VALID_PROPERTIES</field>
+  </difference>
+  
 </differences>