Remove logic for legacy bundled stubs #14325

Job	Run time
Test mypyc suite with 32-bit Python	7m 10s
Test suite with py39-ubuntu, mypyc-compiled	18m 29s
Test suite with py39-windows-64	21m 0s
Test suite with py310-ubuntu	17m 41s
Test suite with py311-ubuntu, mypyc-compiled	16m 8s
Test suite with py312-ubuntu, mypyc-compiled	18m 9s
Test suite with py313-ubuntu, mypyc-compiled	18m 34s
mypyc runtime tests with py39-macos	4m 43s
Type check our own code (py39-ubuntu)	1m 49s
Type check our own code (py39-windows-64)	2m 19s
Formatting and code style with Black + ruff	59s
	2h 7m 1s

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