fix: remove google-cloud-spanner-executor from the BOM (#2844)

Author: suztomo

google-cloud-spanner-bom/pom.xml

@@ -55,11 +55,6 @@
         <artifactId>google-cloud-spanner</artifactId>
         <version>6.56.1-SNAPSHOT</version><!-- {x-version-update:google-cloud-spanner:current} -->
       </dependency>
-      <dependency>
-        <groupId>com.google.cloud</groupId>
-        <artifactId>google-cloud-spanner-executor</artifactId>
-        <version>6.56.1-SNAPSHOT</version><!-- {x-version-update:google-cloud-spanner-executor:current} -->
-      </dependency>
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-spanner</artifactId>