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)

cmake/clang_flags.cmake

@@ -2,8 +2,6 @@
 # -Wno-c++98-compat               The library targets C++11.
 # -Wno-c++98-compat-pedantic      The library targets C++11.
 # -Wno-deprecated-declarations    The library contains annotations for deprecated functions.
-# -Wno-extra-semi-stmt            Vendored Doctest's CAPTURE() macro expands to a
-#                                 statement followed by a semicolon at every call site.
 # -Wno-padded                     We do not care about padding warnings.
 # -Wno-covered-switch-default     All switches list all cases and a default case.
 # -Wno-c2y-extensions             Clang 22.1 diagnoses __COUNTER__ as a C2y extension, also in
@@ -21,7 +19,6 @@ set(CLANG_CXXFLAGS
     -Wno-c++98-compat
     -Wno-c++98-compat-pedantic
     -Wno-deprecated-declarations
-    -Wno-extra-semi-stmt
     -Wno-padded
     -Wno-covered-switch-default
     -Wno-c2y-extensions