Remove codecov badge from README #14684

Job	Run time
sanity checks	12s
python lint check	22s
clang format	35s
rustfmt check	10s
rust build & test	8m 53s
shell test	13m 17s
clang tidy & clangd diagnostics check	22m 33s
generate binary dataset	1m 19s
clang build & test	10m 45s
webassembly build & test	23m 18s
code coverage	32m 15s
msvc build & test	36m 58s
gcc build & test	18m 54s
apple clang build & test	4m 54s
macos extension test	43s
clang build & test with page_size_log2=16	8m 38s
clang build & test with page_size_log2=18	10m 23s
linux extension test	20m 33s
gcc build & test with asan	48m 41s
gcc build & test in-mem only with various vector size	5m 54s
benchmark / benchmark	12m 38s
report benchmark result	6s
windows extension test	7m 51s
	4h 49m 52s

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