fix: remove an invalid per-target key

`sources` is not a per-target key; mcpp reported it as unsupported and ignored
it. It was also unnecessary: with no module interface units present, the target
is inferred from the translation units, which is the intended result.

Author: speak-agent

mcpp.toml

@@ -13,6 +13,3 @@ openkal = "0.2.0"
 # The package contributes definitions and no modules. The interface it
 # implements is declared by the specification package, which this package
 # imports and does not re-export.
-[targets.openkal-linux]
-kind    = "lib"
-sources = ["src/**/*.cpp"]