[Rust] Remove unnecessary mut qualifier in after_member

Author: Vadim Platonov

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/rust/RustGenerator.java

@@ -648,7 +648,7 @@ private static void writeGroupDecoderTopTypes(
             indent(out).append("}\n");
 
             indent(out).append("#[inline]\n");
-            indent(out, 1, "fn after_member(mut self) -> %s {\n", groupLevelNextDecoderType);
+            indent(out, 1, "fn after_member(self) -> %s {\n", groupLevelNextDecoderType);
             indent(out, 2).append("if self.index <= self.max_index {\n");
             indent(out, 3).append("Either::Left(self)\n");
             indent(out, 2).append("} else {\n").append(INDENT).append(INDENT).append(INDENT)