Mk/Uses/cargo.mk: Remove cross-compilation options for Rust-based ports

Reasons:

• Port build shouldn't use cross-compilation mode because it buids for the same architecture.
• RUSTFLAGS isn't passed to the build of all or some Rust ports in the cross-compilation mode.

Essential changes:

• Remove the CARGO_BUILD_TARGET make variable.
• Remove CARGO_BUILD_TARGET and CARGO_TARGET_* from CARGO_ENV.
• Update many ports that used CARGO_BUILD_TARGET.
• Build paths now don't include the architecture triplet part.

Immadiate benefits:

• Unbreak build on architectures like i386 that sometimes require special RUSTFLAGS that were not effective in the cross-compilation mode.

Ports that really need cross-compilation for some reason should
enable it on case-by-case basis. Example: net-p2p/cncli
(Not sure why does net-p2p/cncli actually need it, but it only
builds with these options.)

PR: 280305
Approved by: rust@FreeBSD.org (maintainer's timeout; 98 days)