YJIT: Remove unused src_ctx from Block

[k0kubun]

Pairing with Maxime and Alan, we found that we don't use src_ctx and removing this contributes to reducing memory usages. This PR removes the unused field.

With yjit_alloc_size implemented in #6712, I confirmed it has the following memory usage impact on 25 itrs of railsbench:

Before

inline_code_size:         4948216
outlined_code_size:       4947968
freed_code_size:                0
yjit_alloc_size:         25137225

JIT code: 9.8MB, payload: 25.1MB

After

inline_code_size:         4852292
outlined_code_size:       4850940
freed_code_size:                0
yjit_alloc_size:         23306197

JIT code: 9.7MB, payload: 23.3MB

(*_code_size changes would be just an unrelated fluctuation, but I included it for comparison to yjit_alloc_size)