[minor] remove unused macro definition

[dentiny]

Rationale for this change

I met unused warning at compilation

[~/arrow-rs-object-store] (main) 
vscode@bf21a2fa9d74$ cargo test --lib in_memory_test
   Compiling object_store v0.13.2 (/home/vscode/arrow-rs-object-store)
warning: unused macro definition: `maybe_skip_integration`
    --> src/lib.rs:2157:18
     |
2157 |     macro_rules! maybe_skip_integration {
     |                  ^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: `#[warn(unused_macros)]` on by default

warning: unused import: `maybe_skip_integration`
    --> src/lib.rs:2165:20
     |
2165 |     pub(crate) use maybe_skip_integration;
     |                    ^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: `#[warn(unused_imports)]` on by default

What changes are included in this PR?

This PR removes unused macro, which is complaint by compiler.

Are there any user-facing changes?

No

[crepererum]

I think the code IS used, e.g. here:

arrow-rs-object-store/src/gcp/mod.rs

Line 316 in 0646c6d

maybe_skip_integration!();

[dentiny]

I think the code IS used, e.g. here:

I must get fooled by the warning message! Sorry for the spam, shame on my