Remove _self argument from block trampoline

[tgymnich]

The self argument is not part of the block calling convention [see clang docs]:(https://github.com/JuliaInterop/ObjectiveC.jl/blob/4e8cca652aabe009c90bc01ae92ae8ee87266c30/src/blocks.jl#L98)}

a. The invoke function pointer is set to a function that takes the Block structure as its first argument and the rest of the arguments (if any) to the Blockand executes the Block compound statement.

[maleadt]

LGTM. I guess I was too focussed on implementationWithBlock when implementing this.