Remove semihosting dependency from cortex-r-t - wasn't used

Closes #64

Author: jonathanpallant

cortex-r-rt/Cargo.toml

@@ -25,7 +25,6 @@ version = "0.2.0"
 
 [dependencies]
 cortex-ar = {version = "0.2.0", path = "../cortex-ar"}
-semihosting = {version = "0.1.18", features = ["stdio"]}
 cortex-ar-rt-macros = {path = "../cortex-ar-rt-macros", version = "=0.1.0"}
 
 [features]