Drop the redundant semicolon from CAPTURE() call sites; remove -Wno-extra-semi-stmt

doctest_compatibility.h defines CAPTURE(x) as DOCTEST_CAPTURE(x); (with
a trailing semicolon baked into the macro), specifically so call sites
do not need to add one themselves; most of the ~267 call sites already
follow that convention. The remaining 64 call sites across 20 files
wrote "CAPTURE(x);" anyway, turning into a statement plus an empty
statement and triggering -Wextra-semi-stmt. Dropped the redundant
semicolon at each of those sites.

With item 6 having already made vendored Doctest a SYSTEM include, and
this the last known source of -Wextra-semi-stmt findings, removed the
flag from clang_flags.cmake entirely.

Verified with clang++ -Wextra-semi-stmt (plus the file's other CI
ignores) that all 20 touched files, plus a file with no CAPTURE() use
(unit-json_pointer.cpp), compile without any -Wextra-semi-stmt
diagnostic.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

#5725 item 8 (step 2 of 2)

tests/abi/config/config.hpp

@@ -28,7 +28,7 @@ std::string namespace_name(std::string ns, T* /*unused*/ = nullptr) // NOLINT(pe
     std::smatch m;
 
     // extract the true namespace name from the function signature
-    CAPTURE(ns);
+    CAPTURE(ns)
     CHECK(std::regex_search(ns, m, std::regex("nlohmann(::[a-zA-Z0-9_]+)*::basic_json")));
 
     return m.str();