lang: Remove the fallback function shortcut in try_entry function

[acheroncrypto]

Problem

try_entry function returns early if the instruction data is less than 8 bytes:

anchor/lang/syn/src/codegen/program/entry.rs

Lines 65 to 67 in ccac42d

	if data.len() < 8 {
	return #fallback_maybe;
	}

Summary of changes

• Remove the fallback function shortcut in try_entry function
• Remove gen_fallback function and inline where the fallback is being used (since this is the only place it's being used)
• Remove generate_event_cpi_handler function and inline it to make it consistent with the above change

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Note: This PR is part of a greater effort explained in #3097.

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