Remove workspace when building a contract

[cmichi]

In use-ink/ink#1054 @xgreenx found that if the [workspace] is removed from a contract's Cargo.toml the size decreases potentially significantly.

We should have cargo-contract remove the [workspace] section before building the contract.

[xgreenx]

Adding of empty [workspace] ignores [workspace] from a parent. So it can be a solution=)

[workspace]
members = []

So I can handle that=)