don't set both -enforce-exclusivity=unchecked and none

Exclusivity checking has high runtime performance cost, which is undesirable
for embedded systems. Specifying both options doesn't make sense; opt for
-enforce-exclusivity=unchecked which retains static analysis but disables
runtime checking. [1]

[1] https://www.swift.org/blog/swift-5-exclusivity/

Author: lhoward

classes/swift.bbclass

@@ -150,7 +150,6 @@ python swift_do_configure() {
             "${STAGING_DIR_NATIVE}/usr/bin",
 
             "-enforce-exclusivity=unchecked",
-            "-enforce-exclusivity=none",
 
             "-Xlinker", "-rpath", "-Xlinker", "/usr/lib/swift/linux",