changelog: cleanup 3.0.0-alpha3 changelogs

Remove issues released in 2.11.1 from master (3.0.0-alpha3)

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changelogs/unreleased/ghs-122-allocations-in-printf.md

-## bugfix/sql
-
-* Fixed an integer overflow issue and added check for the `printf()` failure due
-  to too large size (ghs-122).

changelogs/unreleased/ghs-73-unsafe-extensions-decoding.md

-## bugfix/core
-
-* Fixed various bugs related to unsafe (i.e., from an unknown source) decoding
-  and validating of MsgPack extensions (ghs-73).

changelogs/unreleased/luajit-grammar-fuzzer.md

-## feature/test/fuzz
-
-* Now LuaJIT can be fuzzed using grammar-based fuzzer (gh-4823).

changelogs/unreleased/no-gh-add-tt-dependency.md

-## feature/build
-
-* Added `tt` weak dependency. The `tt` command line utility combines
-  `tarantoolctl` and `cartridge-cli` functionality into one tool.
-  The change is a part of pushing `tt` to the public (tarantool/tt#286).

changelogs/unreleased/xlog-reader-show-unknown-header-fields.md

-## bugfix/lua
-
-* Fixed the xlog reader Lua module to show unknown row header fields. Before
-  this change the xlog reader silently skipped them.