readers: remove dead _next_range member from mutation_reader::impl

The `std::optional<dht::partition_range> _next_range` member of
`mutation_reader::impl` is never read or written anywhere in the tree
(verified across the entire codebase); only its declaration exists. Its
comment claims it is "set by fill buffer for segregating output by
partition range", but no such code remains.

Remove the dead member. `std::optional<dht::partition_range>` is 96
bytes, so this shrinks `mutation_reader::impl` -- the base class of
every reader implementation -- by 96 bytes per instance, with no
behavioural change.

Tested: build/dev/test/boost/combined_tests --run_test=mutation_reader_test
(smp=2) passes ("No errors detected").

Signed-off-by: Yaniv Kaul <yaniv.kaul@scylladb.com>
AI-assisted: Yes
Backport: no, improvement

readers/mutation_reader.hh

@@ -137,9 +137,6 @@ public:
         // unless the reader is fast-forwarded to a new range.
         bool _end_of_stream = false;
 
-        // Set by fill buffer for segregating output by partition range.
-        std::optional<dht::partition_range> _next_range;
-
         schema_ptr _schema;
         reader_permit _permit;
         friend class mutation_reader;