include/elist: Fix clear() to use pop_front()

include/elist: Fix clear() to use pop_front()

elist<T>::clear() is calling remove(), which isn't a
method defined on elist<T> (it was never defined according
to git).  Because elist is templated and no references
to clear() are ever made, the compiler matches remove(T) to the
remove(const char *) system call defined in stdio.h.

Once clear is invoked on an instance of elist<T>, we get the
compile error shown below.

The fix here is to use pop_front() instead of remove().

Compile error is:
In file included from ../../src/mds/CInode.h:22:0,
                 from ../../src/mds/CInode.cc:19:
../../src/include/elist.h: In instantiation of ‘void elist<T>::clear() [with T = cinode_backtrace_info_t*]’:
../../src/mds/CInode.cc:1129:20:   required from here
../../src/include/elist.h:101:7: error: no matching function for call to ‘remove(cinode_backtrace_info_t*)’
../../src/include/elist.h:101:7: note: candidates are:
In file included from ../../src/mds/CInode.cc:17:0:
/usr/include/stdio.h:179:12: note: int remove(const char*)
/usr/include/stdio.h:179:12: note:   no known conversion for argument 1 from ‘cinode_backtrace_info_t*’ to ‘const char*’
In file included from /usr/include/c++/4.7/algorithm:63:0,
                 from /usr/include/c++/4.7/backward/hashtable.h:65,
                 from /usr/include/c++/4.7/ext/hash_map:65,
                 from ../../src/include/encoding.h:292,
                 from ../../src/common/entity_name.h:22,
                 from ../../src/common/config.h:26,
                 from ../../src/mds/CInode.h:20,
                 from ../../src/mds/CInode.cc:19:
/usr/include/c++/4.7/bits/stl_algo.h:1117:5: note: template<class _FIter, class _Tp> _FIter std::remove(_FIter, _FIter, const _Tp&)
/usr/include/c++/4.7/bits/stl_algo.h:1117:5: note:   template argument deduction/substitution failed:
In file included from ../../src/mds/CInode.h:22:0,
                 from ../../src/mds/CInode.cc:19:
../../src/include/elist.h:101:7: note:   candidate expects 3 arguments, 1 provided

Signed-off-by: Sam Lang <sam.lang@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>