[libc++] Remove workarounds for the lack of deduction guides in C++17

All supported compilers have supported deduction guides in C++17 for a
while, so this isn't necessary anymore.

Differential Revision: https://reviews.llvm.org/D108213

libcxx/test/std/strings/basic.string/string.cons/iter_alloc_deduction.pass.cpp

@@ -8,7 +8,6 @@
 
 // <string>
 // UNSUPPORTED: c++03, c++11, c++14
-// XFAIL: libcpp-no-deduction-guides
 
 // template<class InputIterator>
 //   basic_string(InputIterator begin, InputIterator end,
@@ -25,7 +24,6 @@
 //  is a type that does not qualify as an input iterator, or if Allocator is a type
 //  that does not qualify as an allocator.
 
-
 #include <string>
 #include <iterator>
 #include <cassert>