Remove unused 'ctxMarker' variables

[tschwinge]

On powerpc64le-unknown-linux-gnu, 'g++ (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0', a
plain 'cmake && make' build fails:

[ 36%] Building CXX object libHSAIL/CMakeFiles/hsail.dir/HSAILScannerRules.cpp.o
In file included from [...]/libHSAIL/HSAILScannerRules.cpp:46:0:
[...]/libHSAIL/generated/HSAILScannerRules_gen_re2c.hpp: In member function 'HSAIL_ASM::ETokens HSAIL_ASM::Scanner::scanModifier(HSAIL_ASM::EScanContext, HSAIL_ASM::Scanner::Token&)':
[...]/libHSAIL/generated/HSAILScannerRules_gen_re2c.hpp:271:17: error: unused variable 'ctxMarker' [-Werror=unused-variable]
     const char *ctxMarker = NULL;
                 ^~~~~~~~~
[...]/libHSAIL/generated/HSAILScannerRules_gen_re2c.hpp: In member function 'HSAIL_ASM::ETokens HSAIL_ASM::Scanner::scanDefault(HSAIL_ASM::EScanContext, HSAIL_ASM::Scanner::Token&)':
[...]/libHSAIL/generated/HSAILScannerRules_gen_re2c.hpp:14065:17: error: unused variable 'ctxMarker' [-Werror=unused-variable]
     const char *ctxMarker = NULL;
                 ^~~~~~~~~
cc1plus: all warnings being treated as errors
libHSAIL/CMakeFiles/hsail.dir/build.make:346: recipe for target 'libHSAIL/CMakeFiles/hsail.dir/HSAILScannerRules.cpp.o' failed
make[2]: *** [libHSAIL/CMakeFiles/hsail.dir/HSAILScannerRules.cpp.o] Error 1
CMakeFiles/Makefile2:87: recipe for target 'libHSAIL/CMakeFiles/hsail.dir/all' failed
make[1]: *** [libHSAIL/CMakeFiles/hsail.dir/all] Error 2
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2

In the generated file 'libHSAIL/generated/HSAILScannerRules_gen_re2c.hpp', the
'libHSAIL/HSAILScannerRules.re2c' change only removes the two 'ctxMarker'
variables, and doesn't cause any other changes, so presumably they're unused,
and can be removed, together with 're2c:define:YYCTXMARKER'.