Remove unused variables in glean/rts/tests/SubstitutionTest.cpp

Summary:
LLVM-15 has a warning `-Wunused-but-set-variable` which we treat as an error because it's so often diagnostic of a code issue. Unused variables can compromise readability or, worse, performance.

This diff either (a) removes an unused variable and, possibly, it's associated code, or (b) qualifies the variable with `[[maybe_unused]]`, mostly in cases where the variable _is_ used, but, eg, in an `assert` statement that isn't present in production code.

 - If you approve of this diff, please use the "Accept & Ship" button :-)

Reviewed By: palmje

Differential Revision: D56022388

fbshipit-source-id: 1ce16bb29d0a9b4f5735122089f8e9c51846a27f

Author: r-barnes

gloo/transport/ibverbs/buffer.cc

@@ -168,8 +168,6 @@ void Buffer::waitSend() {
 }
 
 void Buffer::send(size_t offset, size_t length, size_t roffset) {
-  int rv;
-
   // Can't assert on roffset, since we don't know the size of
   // the remote buffer. Refactor of initialization code needed
   // to support this.