Fix compile: replace foreach with std::remove_if + erase

Instead of using a foreach which operates on a copy and then
mutates the original list, use idiomatic C++ with std::remove_if
and erase. Fixes compile error now that foreach is disallowed:

kidentitymanagement/src/signature.cpp: In member function ‘void KIdentityManagement::SignaturePrivate::cleanupImages()’:
kidentitymanagement/src/signature.cpp:194:18: error: expected primary-expression before ‘const’
          foreach (const Signature::EmbeddedImagePtr &imageInList, embeddedImages) { //Don't use for(...:...) here.