LibTLS: Move singleton for DefaultRootCACertificates out of line

This follows the pattern of every other singleton in the system.

Also, remove use of AK::Singleton in place of a function-scope static.
There are only three uses of that class outside of the Kernel, and all
the remaining uses are suspect. We need it in the Kernel because we
want to avoid global destructors to prevent nasty surprises about
expected lifetimes of objects. In Userland, we have normal thread-safe
statics available. 7d11edbe1 attempted to standardize the pattern, but
it seems like more uses of awkward singleton creation have crept in or
were missed back then.

As a bonus, this fixes a linker error on macOS with -g -O0 for Lagom
WebContent.

Userland/Libraries/LibTLS/Certificate.h

@@ -9,7 +9,6 @@
 #include <AK/ByteBuffer.h>
 #include <AK/Forward.h>
 #include <AK/Optional.h>
-#include <AK/Singleton.h>
 #include <AK/Types.h>
 #include <LibCore/ConfigFile.h>
 #include <LibCore/DateTime.h>
@@ -295,11 +294,9 @@ public:
     static ErrorOr<Vector<Certificate>> parse_pem_root_certificate_authorities(ByteBuffer&);
     static ErrorOr<Vector<Certificate>> load_certificates();
 
-    static DefaultRootCACertificates& the() { return s_the; }
+    static DefaultRootCACertificates& the();
 
 private:
-    static Singleton<DefaultRootCACertificates> s_the;
-
     Vector<Certificate> m_ca_certificates;
 };
 

Userland/Libraries/LibTLS/TLSv12.cpp

@@ -489,7 +489,6 @@ Vector<Certificate> TLSv12::parse_pem_certificate(ReadonlyBytes certificate_pem_
     return { move(certificate) };
 }
 
-Singleton<DefaultRootCACertificates> DefaultRootCACertificates::s_the;
 DefaultRootCACertificates::DefaultRootCACertificates()
 {
     auto load_result = load_certificates();
@@ -501,6 +500,12 @@ DefaultRootCACertificates::DefaultRootCACertificates()
     m_ca_certificates = load_result.release_value();
 }
 
+DefaultRootCACertificates& DefaultRootCACertificates::the()
+{
+    static DefaultRootCACertificates s_the;
+    return s_the;
+}
+
 ErrorOr<Vector<Certificate>> DefaultRootCACertificates::load_certificates()
 {
     auto cacert_file = TRY(Core::File::open("/etc/cacert.pem"sv, Core::File::OpenMode::Read));