Remove __cplusplus from compat/assumptions.h

It is unclear what the goal of this check is, given that the value may
need to be set lower for the mimimum supported version of compilers that
forgot to bump the value, see
bitcoin/bitcoin#28349 (comment) .

The minimum supported compiler versions are already documented in
doc/dependencies.md and using an older compiler will already result in a
compile failure, so this check can be removed as redundant. Especially
given that it is only included in one file, where iwyu suggests to
remove it.

Author: MarcoFalke

src/compat/assumptions.h

@@ -11,13 +11,6 @@
 #include <cstddef>
 #include <limits>
 
-// Assumption: We assume a C++17 (ISO/IEC 14882:2017) compiler (minimum requirement).
-// Example(s): We assume the presence of C++17 features everywhere :-)
-// ISO Standard C++17 [cpp.predefined]p1:
-// "The name __cplusplus is defined to the value 201703L when compiling a C++
-//  translation unit."
-static_assert(__cplusplus >= 201703L, "C++17 standard assumed");
-
 // Assumption: We assume the floating-point types to fulfill the requirements of
 //             IEC 559 (IEEE 754) standard.
 // Example(s): Floating-point division by zero in ConnectBlock, CreateTransaction