upstream: Remove unused variables

[nokute78]

This patch is to address following warinngs.

tests/internal/config_format_fluentbit.c:

[ 95%] Building C object tests/internal/CMakeFiles/flb-it-config_format_fluentbit.dir/config_format_fluentbit.c.o
/home/taka/git/fluent-bit/tests/internal/config_format_fluentbit.c: In function ‘test_nolimit_line’:
/home/taka/git/fluent-bit/tests/internal/config_format_fluentbit.c:265:21: warning: unused variable ‘head’ [-Wunused-variable]
  265 |     struct mk_list *head;
      |                     ^~~~

flb_upstream.c:

[ 70%] Building C object src/CMakeFiles/fluent-bit-shared.dir/flb_upstream.c.o
/home/taka/git/fluent-bit/src/flb_upstream.c: In function ‘shutdown_connection’:
/home/taka/git/fluent-bit/src/flb_upstream.c:445:26: warning: variable ‘u’ set but not used [-Wunused-but-set-variable]
  445 |     struct flb_upstream *u;
      |                          ^

input_chunk.c:

[ 68%] Building C object src/CMakeFiles/fluent-bit-shared.dir/flb_input_chunk.c.o
/home/taka/git/fluent-bit/src/flb_input_chunk.c: In function ‘input_chunk_get’:
/home/taka/git/fluent-bit/src/flb_input_chunk.c:1034:12: warning: unused variable ‘expected_chunk_size’ [-Wunused-variable]
 1034 |     size_t expected_chunk_size;
      |            ^~~~~~~~~~~~~~~~~~~

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Testing
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• [N/A] Example configuration file for the change
• [N/A] Debug log output from testing the change

• [N/A] Attached Valgrind output that shows no leaks or memory corruption was found

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• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.