Remove basic_json proxy #3165

Job	Run time
ci_test_compilers_clang (3.8)	25s
ci_test_compilers_clang (3.9)	25s
ci_test_compilers_clang2 (13)	23s
ci_test_compilers_clang (4)	24s
ci_test_compilers_gcc (8)	23s
ci_test_compilers_clang (5)	23s
ci_test_compilers_gcc2 (12)	23s
ci_test_compilers_clang2 (15-bullseye)	26s
ci_test_compilers_gcc (9)	22s
ci_test_compilers_clang (6)	23s
ci_test_compilers_gcc2 (latest)	28s
ci_test_compilers_gcc (10)	27s
ci_test_compilers_clang (7)	22s
clang_14_build	11s
ci_test_compilers_gcc (11)	26s
ci_test_compilers_clang (8)	24s
ci_test_compilers_gcc (12)	23s
ci_test_compilers_clang (9)	23s
gcc_build	10s
ci_test_compilers_gcc (latest)	21s
gcc_build_no_deprecated	12s
ci_test_compilers_clang (10)	22s
gcc_build11b	10s
ci_test_compilers_clang (11)	25s
github_20 (g++-12, Debug)	8s
ci_test_compilers_clang (12)	24s
gcc_build12	11s
github_20 (g++-12, Release)	8s
ci_test_compilers_clang (13)	25s
ci_test_compilers_clang (14)	21s
ci_test_compilers_clang (15-bullseye)	24s
	10m 42s

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