Remove unneeded art::Unit member functions

Remove ~Unit to avoid a build failure after libc++ upgrade:

art/cmdline/unit.h:29:3: error: definition of implicit copy assignment operator for 'Unit' is deprecated because it has a user-provided destructor [-Werror,-Wdeprecated-copy-with-user-provided-dtor]

(Perhaps std::vector changed to use assignment instead of a copy ctor.)

Also remove the Unit() default ctor, because it's unnecessary, and
Valgrind support has been deprecated for years now. Similarly, remove
the explicitly-defaulted copy ctor. It can be defined implicitly (i.e.
following the C++ "rule of zero").

Bug: http://b/175635923
Test: treehugger
Change-Id: Iefd41dad5b88361104ab159549058daa3b895612
diff --git a/cmdline/unit.h b/cmdline/unit.h
index b049d62..0b5c344 100644
--- a/cmdline/unit.h
+++ b/cmdline/unit.h
@@ -21,12 +21,6 @@
 
 // Used for arguments that simply indicate presence (e.g. "-help") without any values.
 struct Unit {
-  // Historical note: We specified a user-defined constructor to avoid
-  // 'Conditional jump or move depends on uninitialised value(s)' errors
-  // when running Valgrind.
-  Unit() {}
-  Unit(const Unit&) = default;
-  ~Unit() {}
   bool operator==(const Unit&) const {
     return true;
   }