default.conf: remove CARGO_REGISTRIES_CRATES_IO_PROTOCOL

`sparse` is the default protocol for accessing crates.io since 2023
March.

Ref: rust-lang/cargo#11791

Author: fossdd

default.conf

@@ -17,7 +17,6 @@ export CARGO_PROFILE_RELEASE_OPT_LEVEL="s"
 export CARGO_PROFILE_RELEASE_PANIC="abort"
 export CARGO_PROFILE_RELEASE_CODEGEN_UNITS=1
 export CARGO_PROFILE_RELEASE_LTO="true"
-export CARGO_REGISTRIES_CRATES_IO_PROTOCOL="sparse"
 
 export CTEST_OUTPUT_ON_FAILURE=ON
 export CTEST_NO_TESTS_ACTION=error