Use the stdlib more instead of defining methods in `dotc.util`

[hamzaremmal]

• use methods that are already defined in the stdlib (11e426653a775de7cfca1d3f6886ac99f315964a, c6fe60570085f7a23b1a9dff6f90d85c9340cf31)
• remove empty files (3bddc16c9fb0d25632a694fe1c16e0ad3b17f5f8)