Remove DLSAG and MDLSAG implementations

This PR removes both the DLSAG (Dual Linkable Spontaneous Anonymous Group) and MDLSAG (Multilayer Dual Linkable Spontaneous Anonymous Group) signature schemes as their implementations did not correctly match the specifications in the cited paper (https://eprint.iacr.org/2019/595.pdf).

Major Changes:

• Removed src/dlsag.rs and src/mdlsag.rs files
• Removed DLSAG and MDLSAG module references from src/lib.rs
• Removed DLSAG and MDLSAG tests from tests/serde_tests.rs and tests/no_std_tests.rs
• Updated documentation in lib.rs and README.md to note the removals
• Bumped version to 2.0.0 due to breaking changes

The core functionality of the following signature schemes remains unchanged:

• SAG (Spontaneous Anonymous Group)
• bLSAG (Back's Linkable Spontaneous Anonymous Group)
• MLSAG (Multilayer Linkable Spontaneous Anonymous Group)
• CLSAG (Concise Linkable Spontaneous Anonymous Group)

Breaking Changes:

• DLSAG and MDLSAG types and traits are no longer available
• Applications using these schemes will need to find alternative implementations
• Version bumped to 2.0.0 in accordance with semver

This change improves the library's accuracy by removing implementations that did not correctly follow the paper's specifications. Users who need DLSAG functionality should refer to the original paper for correct implementation details.