Fix unused variable warning in multi_buffer

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

CHANGELOG.md

@@ -4,6 +4,7 @@ Version 264:
 * Fix unused variable warnings in tests
 * Fix missing initializer warning in `basic_fields`
 * Remove unused functions in `impl/static_string.hpp`
+* Fix unused variable warning in `multi_buffer`
 
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include/boost/beast/core/impl/multi_buffer.hpp

@@ -956,7 +956,6 @@ consume(size_type n) noexcept
 {
     if(list_.empty())
         return;
-    auto a = rebind_type{this->get()};
     for(;;)
     {
         if(list_.begin() != out_)