Revert `mount_path` parameter from `FastMCP` #4110

Job	Run time
checks / readme-snippets	13s
checks / pre-commit	35s
checks / test (3.12, lowest-direct, windows-latest)	3m 29s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 9s
checks / test (3.11, lowest-direct, windows-latest)	3m 4s
checks / test (3.14, lowest-direct, windows-latest)	2m 21s
checks / test (3.10, lowest-direct, ubuntu-latest)	49s
checks / test (3.11, locked, windows-latest)	2m 24s
checks / test (3.13, locked, windows-latest)	2m 25s
checks / test (3.13, lowest-direct, ubuntu-latest)	45s
checks / test (3.10, locked, ubuntu-latest)	42s
checks / test (3.11, lowest-direct, ubuntu-latest)	47s
checks / test (3.13, locked, ubuntu-latest)	43s
checks / test (3.10, locked, windows-latest)	2m 27s
checks / test (3.12, lowest-direct, ubuntu-latest)	55s
checks / test (3.14, locked, windows-latest)	4m 1s
checks / test (3.13, lowest-direct, windows-latest)	2m 28s
checks / test (3.10, lowest-direct, windows-latest)	2m 24s
checks / test (3.11, locked, ubuntu-latest)	43s
checks / test (3.12, locked, windows-latest)	3m 25s
checks / test (3.14, locked, ubuntu-latest)	2m 20s
checks / test (3.12, locked, ubuntu-latest)	49s
	38m 58s

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