Nimble: don't always build library for distribution.

Recently, Nimble moved to using 'CwlPreconditionTesting' not as a
direct compilation, but through SPM. This means they have different
build settings now.

Since Nimble is always built for distribution, but 'CwlPreconditionTesting'
doesn't, the following warning is show:
`Module 'CwlPreconditionTesting' was not compiled with library evolution support; using it means binary compatibility for 'Nimble' can't be guaranteed`.

Since building for distribution is needed mostly when building binaries
it can be simply removed from the project settings and should be enabled
whenever it's built for release.

Nimble.xcodeproj/project.pbxproj

@@ -1179,7 +1179,6 @@
 		1F925EC1195C0D6300ED456B /* Debug */ = {
 			isa = XCBuildConfiguration;
 			buildSettings = {
-				BUILD_LIBRARY_FOR_DISTRIBUTION = YES;
 				COMBINE_HIDPI_IMAGES = YES;
 				DEFINES_MODULE = YES;
 				DYLIB_COMPATIBILITY_VERSION = 1;
@@ -1220,7 +1219,6 @@
 		1F925EC2195C0D6300ED456B /* Release */ = {
 			isa = XCBuildConfiguration;
 			buildSettings = {
-				BUILD_LIBRARY_FOR_DISTRIBUTION = YES;
 				COMBINE_HIDPI_IMAGES = YES;
 				DEBUG_INFORMATION_FORMAT = "dwarf-with-dsym";
 				DEFINES_MODULE = YES;